ath79: add support for Ubiquiti PowerBeam M2 (XW)

ath79: add support for Ubiquiti PowerBeam M2 (XW)

This patch adds support for the Ubiquiti PowerBeam M2 (XW), e.g. PBE-M2-400,
a 802.11n wireless with a feed+dish form factor. This device was previously
supported by the ar71xx loco-m-xw firmware.

Specifications:
 - Atheros AR9342 SoC
 - 64 MB RAM
 - 8 MB SPI flash
 - 1x 10/100 Mbps Ethernet port, 24 Vdc PoE-in
 - Power and LAN green LEDs
 - 4x RSSI LEDs (red, orange, green, green)
 - UART (115200 8N1)

Flashing via stock GUI:
 - Downgrade to AirOS v5.5.x (latest available is 5.5.10-u2) first (see
   https://openwrt.org/toh/ubiquiti/powerbeam installation instructions)
 - Upload the factory image via AirOS web GUI.

Flashing via TFTP:
 - Use a pointy tool (e.g., unbent paperclip) to keep the
   reset button pressed.
 - Power on the device (keep reset button pressed).
 - Keep pressing until LEDs flash alternatively LED1+LED3 =>
   LED2+LED4 => LED1+LED3, etc.
 - Release reset button.
 - The device starts a TFTP server at 192.168.1.20.
 - Set a static IP on the computer (e.g., 192.168.1.21/24).
 - Upload via tftp the factory image:
    $ tftp 192.168.1.20
    tftp> bin
    tftp> trace
    tftp> put openwrt-ath79-generic-ubnt_powerbeam-m2-xw-squashfs-factory.bin

WARNING: so far, no non-destructive method has been discovered for
opening the enclosure to reach the serial console. Internal photos
are available here: https://fcc.io/SWX-NBM2HP

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