ramips: add support for DomyWifi DM202/DM203/DW22D

Specifications:
* SOC: MT7620A + MT7610E
* ROM: 16 MiB spi flash (W25Q128FVSG)
* RAM: 128 MiB DDR2 (W971GG6KB-25)
* WAN: 10/100M *1
* LAN: 10/100M *4
* USB: Type-A USB2.0 *1
* SD: MicroSD *1
* Button: Reset *1
* Antennas: 2.4 GHz *2 + 5 GHz *1
* TTL Baudrate: 57600
* U-Boot Recovery: IP: 10.10.10.123, Server: 10.10.10.3

Installation:
* Web UI Update
  1. Open http://192.168.10.1/upgrade.html in the browser.
  2. Rename firmware to a short name like firmware.bin and then upload it.
  3. Fill in the password column with the following content:
  password | mtd -x mIp2osnRG3qZGdIlQPh1 -r write /tmp/firmware.bin firmware
* TFTP + U-Boot
  1. Connect device with a TTL cable.
  2. Press "2" when booting to select "Load system code then write to Flash via TFTP".
  3. Upload firmware by tftpd64, it will boot when write instruction is executed.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>