ipq806x: add support for Ubiquiti UniFi AC HD

ipq806x: add support for Ubiquiti UniFi AC HD

Hardware
--------

SoC:   Qualcomm IPQ8064
RAM:   512MB DDR3
Flash: 256MB NAND (Micron MT29F2G08ABBEAH4)
       32MB SPI-NOR (Macronix MX25U25635F)
WLAN:  Qualcomm Atheros QCA9994 4T4R b/g/n
       Qualcomm Atheros QCA9994 4T4R a/n/ac
ETH:   eth0 - SECONDARY (Atheros AR8033)
       eth1 - MAIN (Atheros AR8033)
USB:   USB-C
LED:   Dome (white / blue)
BTN:   Reset

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

Copy the OpenWrt sysupgrade image to the /tmp directory of the device
using scp. Default IP address is 192.168.1.20 and default username and
password are "ubnt".

SSH to the device and write the bootselect flag to ensure it is booting
from the mtd partition the OpenWrt image will be written to. Verify the
output device below matches mtd partition "bootselect" using /proc/mtd.

> dd if=/dev/zero bs=1 count=1 seek=7 conv=notrunc of=/dev/mtd11

Write the OpenWrt sysupgrade image to the mtd partition labeled
"kernel0". Also verify the used partition device using /proc/mtd.

> dd if=/tmp/sysupgrade.bin of=/dev/mtdblock12

Reboot the device.

Back to stock
-------------

Use the TFTP recovery procedure with the Ubiquiti firmware image to
restore the vendor firmware.

Signed-off-by: Jan Alexander <jan@nalx.net>