ramips: Add support for Phicomm K2G

ramips: Add support for Phicomm K2G

Specification:
- SoC: MediaTek MT7620A
- Flash: 8 MB
- RAM: 64 MB
- Ethernet: 4 FE ports and 1 GE port (RTL8211F on port 5)
- Wireless radio: MT7620 for 2.4G and MT7612E for 5G, both equipped with external PA.
- UART: 1 x UART on PCB - 57600 8N1

Flash instruction:
The U-boot is based on Ralink SDK so we can flash the firmware using UART:
1. Configure PC with a static IP address and setup an TFTP server.
2. Put the firmware into the tftp directory.
3. Connect the UART line as described on the PCB.
4. Power up the device and press 2, follow the instruction to
   set device and tftp server IP address and input the firmware
   file name. U-boot will then load the firmware and write it into
   the flash.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>