ramips: mt7621: add support for Maginon MC-1200AC

ramips: mt7621: add support for Maginon MC-1200AC

This commit adds support for Maginon MC-1200AC.

Hardware specifications:

SoC: MediaTek MT7621
Flash: 16 MB SPI Flash
RAM: 128 MB RAM
Ethernet:
2x 1G RJ45 ports
WLAN:
2.4GHz: MediaTek MT7603E
5GHz: MediaTek MT7613BE
LEDs: Red and blue status lights
Power: 12V DC
UART: 3.3V, 115200 baud, 8N1, like printed on silkscreen (GND,TX,RX,3.3V)

MAC addresses
-------------
+---------+-------------------+
|         | MAC example       |
+---------+-------------------+
| LAN     | 80:3F:5D:xx:xx:72 |
| WAN     | 80:3F:5D:xx:xx:73 |
| WLAN 2g | 80:3F:5D:xx:xx:74 |
| WLAN 5g | 80:3F:5D:xx:xx:75 |
+---------+-------------------+

Installation:

The firmware can be flashed via the U-Boot recovery web interface.
To access it, hold the reset button while powering on the device.
U-Boot recovery web interface is then avaiable at 192.168.10.1.

Alternatively, the image can be loaded using the U-Boot serial interface and TFTP.

Signed-off-by: Simon Etzlstorfer <simon@etzi.at>
Link: https://github.com/openwrt/openwrt/pull/17671
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>