ramips: add support for Wavlink WL-WN531G3

ramips: add support for Wavlink WL-WN531G3

This device is similiar to the Wavlink WL-WN531A3.
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.
In my case the whole device was locked and there was no way
to flash the image, except for flashing directly to the flash
via an spi-flasher. You need to put the sysupgrade image file at
the beginning of 0x60000.

Notes
-----
1) Router mac addresses:
   LAN XX:XX:XX:XX:XX:F0 (factory @ 0x28)
   WAN XX:XX:XX:XX:XX:F1 (factory @ 0x2e)
   WIFI 2G XX:XX:XX:XX:XX:F2 (factory @ 0x04)
   WIFI 5G XX:XX:XX:XX:XX:F3 (factory @ 0x8004)

   LABEL XX:XX:XX:XX:XX:F2

Signed-off-by: Eros Brigmann <erosbrigmann@gmail.com>