ath79: Add support for Ubiquiti Bullet AC

ath79: Add support for Ubiquiti Bullet AC

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:       1x 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)

Flashing from serial console
1. Connect serial console (115200 baud)
2. Connect ethernet to a network with a TFTP server, through a
   passive PoE injector.
3. Press a key to obtain a u-boot prompt
4. Set your TFTP server's ip address, with:
   setenv serverip <tftp-server-address>
5. Set the Bullet AC's ip address, with:
   setenv ipaddr <bullet-ac-address>
6. Set the boot file, with:
   setenv bootfile <name-of-initramfs-binary-on-tftp-server>
7. Fetch the binary with tftp:
   tftpboot
8. Boot the initramfs binary:
   bootm
9. From the initramfs, fetch the sysupgrade binary, and flash it with
   sysupgrade.

The Bullet AC is identified as a 2WA board by Ubiquiti. As such, the UBNT_TYPE
must match from the "Flashing through factory firmware" install instructions
to work.

Phy0 is QCA988X which can tune either band (2.4 or 5GHz). Phy1 is AR9342,
on which 5GHz is disabled.  It isn't currently known whether phy1 is
routed to the N connector at all.

Signed-off-by: Russell Senior <russell@personaltelco.net>