I decided to increase the offsets to calibration data and MAC address by 0x2000. The device is running fine. According to this:
[ 0.810611] Scanning device for bad blocks
[ 1.286029] Bad eraseblock 368 at 0x000002e00000
[ 1.436596] Bad eraseblock 480 at 0x000003c00000
[ 1.516159] Bad eraseblock 536 at 0x000004300000
[ 2.142906] 6 fixed-partitions partitions found on MTD device mt7621-nand
[ 2.156425] Creating 6 MTD partitions on "mt7621-nand":
[ 2.166843] 0x000000000000-0x000000100000 : "u-boot"
[ 2.178167] 0x000000100000-0x000000200000 : "SC PID"
[ 2.189460] 0x000000200000-0x000000600000 : "kernel"
[ 2.200703] 0x000000600000-0x000002200000 : "ubi"
[ 2.211587] 0x000002e00000-0x000002f00000 : "factory"
[ 2.222985] 0x000004200000-0x000007e00000 : "reserved"
the factory partition is located at 0x2e20000.
Thanks for help.