Adding support for Mercusys MR90X

Stock layout

0x000000000000-0x000000200000 : "boot"        0x200000
0x000000200000-0x000000300000 : "u-boot-env"  0x100000
0x000000300000-0x000003500000 : "ubi0"        0x3200000
0x000003500000-0x000006700000 : "ubi1"        0x3200000
0x000006700000-0x000006f00000 : "userconfig"  0x800000
0x000006f00000-0x000007300000 : "tp_data"     0x400000

"boot"

MT7986> version
U-Boot 2022.01-rc4 (Sep 15 2022 - 20:55:22 +0800)

aarch64-openwrt-linux-musl-gcc.bin (OpenWrt GCC 8.4.0 r0-c067e3725) 8.4.0
GNU ld (GNU Binutils) 2.34

"ubi0" format

+---------------------+--------------------+---------------------+---------------------+
| UBI Image:          |                    |                     |                     |
| Volume Name: uboot  | Volume Name: uboot | Volume Name: kernel | Volume Name: rootfs |
| Volume Name: kernel | Vol ID: 0          | Vol ID: 1           | Vol ID: 2           |
| Volume Name: rootfs | Vol_type: 'static' | Vol_type: 'static'  | Vol_type: 'static'  |
+---------------------+--------------------+---------------------+---------------------+

uboot - secondary u-boot in UBI

ubireader_display_info ubi0
UBI File
---------------------
        Min I/O: 2048
        LEB Size: 126976
        PEB Size: 131072
        Total Block Count: 400
        Data Block Count: 200
        Layout Block Count: 2
        Internal Volume Block Count: 0
        Unknown Block Count: 198
        First UBI PEB Number: 0

        Image: 302291140
        ---------------------
                Image Sequence Num: 302291140
                Volume Name:uboot
                Volume Name:kernel
                Volume Name:rootfs
                PEB Range: 2 - 399

                Volume: uboot
                ---------------------
                        Vol ID: 0
                        Name: uboot
                        Block Count: 6

                        Volume Record
                        ---------------------
                                alignment: 1
                                crc: '0xaea9197b'
                                data_pad: 0
                                errors: ''
                                flags: 0
                                name: 'uboot'
                                name_len: 5
                                padding: '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                                rec_index: 0
                                reserved_pebs: 6
                                upd_marker: 0
                                vol_type: 'static'


                Volume: kernel
                ---------------------
                        Vol ID: 1
                        Name: kernel
                        Block Count: 26

                        Volume Record
                        ---------------------
                                alignment: 1
                                crc: '0xca6c7adb'
                                data_pad: 0
                                errors: ''
                                flags: 0
                                name: 'kernel'
                                name_len: 6
                                padding: '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                                rec_index: 1
                                reserved_pebs: 26
                                upd_marker: 0
                                vol_type: 'static'


                Volume: rootfs
                ---------------------
                        Vol ID: 2
                        Name: rootfs
                        Block Count: 168

                        Volume Record
                        ---------------------
                                alignment: 1
                                crc: '0xe75eee88'
                                data_pad: 0
                                errors: ''
                                flags: 0
                                name: 'rootfs'
                                name_len: 6
                                padding: '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                                rec_index: 2
                                reserved_pebs: 168
                                upd_marker: 0
                                vol_type: 'static'