ath79: add support for Ubiquiti AirRouter

Indoor low-power router with 2.4 GHz radio

CPU:    Atheros AR7241 rev 1
RAM:    32 MB
Flash:  8 MB NOR SPI
Switch: Atheros AR7240
Ports:  1x WAN, 4x LAN 10/100 Ethernet
WLAN:   Atheros AR9285 (2.4 GHz)
USB:    1x USB2 host port

Note: Ethernet WAN/LAN port naming is reversed from ar71xx.
WAN is eth0; LAN is eth1.1.

UART settings: 115200, 8N1

                LEDs
        +--------------------------
        |
        |
        |
        |
        |
        |
        |
        |
    VCC | x x
     RX | * x
        | x x
        | x x
     TX | * x
    GND | * x
        |
        |
        |
        |
        +--------------------------
                ETHERNET PORTS

Installation from Ubiquiti firmware, is as for other ubnt-xm AirOs devices.

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