ramips: add support for Ubiquiti UniFi nanoHD

ramips: add support for Ubiquiti UniFi nanoHD

Hardware
--------
SoC:   MediaTek MT7621AT
WiFi:  MediaTek MT7603 bgn 2T2R
       MediaTek MT7615 ac  4T4R
Flash: 32M SPI (Macronix MX25L25635F)
RAM:   128M DDR3 (Winbond W631GG6KB)
LED:   Dome (Blue / White)
BTN:   Reset

Installation
------------

These instructions were written for firmware version v3.9.27.
Downgrade if necessary.

1. Copy the OpenWrt sysupgrade image to the devices /tmp folder
   via scp. On factory defaults, user and password is "ubnt" at
   192.168.1.20/24.

2. Write the bootselect flag. Otherwise, the device might boot from the
   wrong partition. Verify the mtd partition used in the command below
   is the one labled "bs" in /proc/mtd (as this might change in the
   future).

   > dd if=/dev/zero bs=1 count=1 of=/dev/mtd4

3. Write the OpenWrt sysupgrade to the mtd partitions labled
   "kernel0" and "kernel1".

   > dd if=/tmp/openwrt-sysupgrade.bin of=/dev/mtdblock6
   > dd if=/tmp/openwrt-sysupgrade.bin of=/dev/mtdblock7

4. Reboot or powercycle the device.

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