ath79: Add support for Ubiquiti NanoBeam AC Gen2

ath79: Add support for Ubiquiti NanoBeam AC Gen2

CPU:         Atheros AR9342 rev 3 SoC
RAM:         64 MB DDR2
Flash:       16 MB NOR SPI
WLAN 2.4GHz: Atheros AR9342 v3 (ath9k)
WLAN 5.0GHz: QCA988X
Ports:       2x GbE

Flashing procedure is identical to other ubnt devices.
https://openwrt.org/toh/ubiquiti/common

Flashing through factory firmware
1. Ensure firmware version v8.7.0 is installed.
   Up/downgrade to this exact version.
2. Patch fwupdate.real binary using
   `hexdump -Cv /bin/ubntbox | sed 's/14 40 fe 27/00 00 00 00/g' | \
    hexdump -R > /tmp/fwupdate.real`
3. Make the patched fwupdate.real binary executable using
   `chmod +x /tmp/fwupdate.real`
4. Copy the squashfs factory image to /tmp on the device
5. Flash OpenWrt using `/tmp/fwupdate.real -m <squashfs-factory image>`
6. Wait for the device to reboot
(copied from Ubiquiti NanoBeam AC and modified)

To keep it consistent, we will add the gen1 variant to
the nanobeam ac gen1.

Signed-off-by: Nick Hainke <vincent@systemli.org>