ath79: add support for Ubiquiti NanoBridge M (XM)

ath79: add support for Ubiquiti NanoBridge M (XM)

This patch adds support for the Ubiquiti NanoBridge M (XM), a
802.11n wireless with a feed+dish form factor, with the same board
definition as the Bullet M (XM).

Specifications:
 - Atheros AR7241 SoC
 - 32 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:
 - WARNING: flashing OpenWrt from AirOS v5.6 or newer will brick your
   device! Read the wiki for more info.
 - Downgrade to AirOS v5.5.x (latest available is 5.5.11) first.
 - Upload the factory image via AirOS web GUI.

Flashing via TFTP:
 - WARNING: flashing OpenWrt from AirOS v5.6 or newer will brick your
   device! Read the wiki for more info.
 - Downgrade to AirOS v5.5.x (latest available is 5.5.11) first.
 - Use a pointy tool (e.g., pen cap, slotted screwdriver) 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-xxxxx-ubnt_nanobridge-m-squashfs-factory.bin

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[rebase, fix includes in DTS, add label MAC address, add SOC and
fix sorting in generic-ubnt.mk]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>