ramips: add support for ALFA Network AC1200RM

ramips: add support for ALFA Network AC1200RM

ALFA Network AC1200RM is an AC1200 router, with 5-port FE switch and
USB 2.0 port. Device is based on MediaTek MT7620A + MT7612EN.

Specification:

- MT7620A (580 MHz)
- 64 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 5x 10/100 Mbps Ethernet with passive PoE output in WAN and LAN4
- 2T2R 2.4 GHz (MT7620A)
- 2T2R 5 GHz (MT7612EN)
- 1x USB 2.0
- 9x LED (8 driven by GPIO)
- 1x button (reset)
- DC jack for main power input (12-24 V)
- 2x UART, I2C, I2S and LED headers

Flash instruction (do it under U-Boot, using UART and TFTP server):

Select option "2: Load system code then write to Flash via TFTP" and
use "sysupgrade" image.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>