Add OpenWrt support for Xiaomi "Redmi AX6000"

[redchenjs]

1. The LED panel is attatched to spi bus as spi1.0:

root@XiaoQiang:~# ls /sys/class/spi_master/spi1/spi1.0/leds/
led_blue    led_green   led_red     led_white   led_yellow

2. You can manually set the LED color through sysfs, just use the echo command:

root@XiaoQiang:~# ls /sys/class/spi_master/spi1/spi1.0/leds/led_blue/
brightness      device          max_brightness  power           subsystem       trigger         uevent
root@XiaoQiang:~# echo 1 > /sys/class/spi_master/spi1/spi1.0/leds/led_blue/brightness

3. It looks like the LED panel contains 5 addressable LEDs in series, the control timing is like this (similar to WS2801, need further testing):