ramips: add support for GL.iNet VIXMINI

ramips: add support for GL.iNet VIXMINI

Hardware
--------
SoC:   MediaTek MT7628NN
RAM:   64M DDR2 (Etron EM68B16CWQD-25H)
FLASH: 8M (Winbond W25Q64JVSIQ)
LED:   Power - WLAN
BTN:   Reset
UART:  115200 8N1
       TX and RX are labled on the board as pads next to the SoC

Installation via web-interface
------------------------------
1. Visit the web-interface at 192.168.8.1
   Note: The ethernet port is by default WAN. So you need to connect to
   the router via WiFi

2. Navigate to the Update tab on the left side.

3. Select "Local Update"

4. Upload the OpenWrt sysupgrade image.
   Note: Make sure you select not to preserve the configuration.

Installation via U-Boot
-----------------------
1. Hold down the reset button while powering on the device.
   Wait for the LED to flash 5 times.

2. Assign yourself a static IPv4 in 192.168.1.0/24

3. Upload the OpenWrt sysupgrade image at 192.168.1.1.

Signed-off-by: David Bauer <mail@david-bauer.net>