ath79: add support for Ubiquiti NanoStation Loco M (XW)

ath79: add support for Ubiquiti NanoStation Loco M (XW)

This commit adds support for the NanoStation Loco M2/M5 XW devices
on the ath79 target (support was long ago available on ar71xx).

Specifications:

 - AR9342 SoC @ 535 MHz
 - 64 MB RAM
 - 8 MB SPI flash
 - 1x 10/100 Mbps Ethernet, 24 Vdc PoE-in
 - AR8032 switch
 - 2T2R 5 GHz radio, 22 dBm
 - 13 dBi built-in antenna
 - POWER/LAN green LEDs
 - 4x RSSI LEDs (red, orange, green, green)
 - UART (115200 8N1) on PCB

Flashing via TFTP:

 - Use a pointy tool (e.g., pen cap, paper clip) and keep the reset
   button on the device or on the PoE supply pressed
 - Power on the device via PoE (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_nanostation-loco-m-xw-squashfs-factory.bin

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>