ramips: add support for Phicomm K2P

ramips: add support for Phicomm K2P

It uses one MT7615D radio chip with DBDC mode enabled. This mode allows
this single chip act as an 2x2 11n radio and an 2x2 11ac radio at the
same time. However mt76 doesn't  support it currently so there is no
wireless available.

Specification:
- SoC: MediaTek MT7621AT
- Flash: 16 MB
- RAM: 128 MB
- Ethernet: 1 x WAN (10/100/1000Mbps) and 4 x LAN (10/100/1000 Mbps)
- Wireless radio: MT7615D on PCIE0
- UART: 1 x UART on PCB - 57600 8N1

Issue:
- Wireless radio doesn't work due to the lack of driver.

Flash instruction:
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,then 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>