ramips: mt7620: Add support for the devolo WiFi Repeater ac (MT 2767, 2779)

ramips: mt7620: Add support for the devolo WiFi Repeater ac (MT 2767, 2779)

Specification:
    - MT7620A 580 MHz MIPS24KEc
    - 64MB RAM
    - 8MB SPI NOR
    - MediaTek MT7612E 5.0GHz 802.11a/n/ac
    - MediaTek MT7620 2.4GHz 802.11b/g/n
    - 5 LEDs (white)
    - 1 button
    - 1 Gbit port Realtek RTL8211E GbE Phy

Serial Interface:
    - 3 Pins GND, RX, TX
    - Settings: 57600, 8N1

Based on support from edimax_ew-7476rpc/edimax_ew-747x
and netgear_ex3700/netgear_ex3x00_ex61xx

Notes:
 - ATM there is no known way to revert to stock firmware

Flash instruction:
The only known way to flash OpenWrt image is to use tftp in U-Boot, with the
aid of a serial adapter for U-Boot console access:

1. Open the device and connect to the serial port. The device is very similar
   to Edimax 7476RPC. See https://openwrt.org/toh/edimax/ew-7476rpc. No VCC!
2. Configure PC with static IP 192.168.1.2/24 and tftp server and
   connect PC to device using an ethernet cable.
3. Power on the device and, on the serial console, as soon as U-Boot starts
   loading, press "2" to interrupt loading.
4. Enter device ip address 192.168.1.1, PC ip address 192.168.1.2
   and the firmware filename placed on the TFTP server.
5. Device will download file from server, write it to flash and reboot.

Signed-off-by: Hugo Monteiro <monteiro.hugo@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16956
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>