ramips: add support for Wavlink WL-WN531G3-A2

ramips: add support for Wavlink WL-WN531G3-A2

This device is exactly the same as WL-WN531G3 but with different partition layout and different MAC layout. Labeled as Quantum D4G Rev.: A2.

Hardware
--------
SoC:   Mediatek MT7620A
RAM:   64MB
FLASH: 8MB NOR (GigaDevice GD25Q64CS)
ETH:
  - 2x 10/100/1000 Mbps Ethernet (RTL8211F)
  - 3x 10/100 Mbps Ethernet (integrated in SOC)
WIFI:
  - 2.4GHz: 1x (integrated in SOC) (2x2:2)
  - 5GHz:   1x MT7612E (2x2:2)
  - 4 external antennas
BTN:
  - 1x Reset button
  - 1x Touchlink button
  - 1x Turbo button
  - 1x Wps button
  - 1x ON/OFF switch
LEDS:
  - 1x Red led (system status)
  - 1x Blue led (system status)
  - 5x Blue leds (ethernet ports)
  - 1x Power led
  - 1x Wifi led
UART:
  - 57600-8-N-1
Everything works correctly.

Installation
------------
Flash the initramfs image in the OEM firmware interface
When Openwrt boots, flash the sysupgrade image otherwise you won't be
able to keep configuration between reboots.

Notes
-----
1) Router mac addresses:
   LAN XX:XX:XX:XX:XX:0F (factory @ 0x28)
   WAN XX:XX:XX:XX:XX:10 (factory @ 0x2e)
   WIFI 2G XX:XX:XX:XX:XX:11 (factory @ 0x04)
   WIFI 5G XX:XX:XX:XX:XX:12 (factory @ 0x8004)

   LABEL XX:XX:XX:XX:XX:11

Signed-off-by: Eros Brigmann <erosbrigmann@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15876
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>