ar71xx: Add support for Ubiquity Bullet M (XW)

ar71xx: Add support for Ubiquity Bullet M (XW)

CPU: AR9342 SoC
RAM:     64 MB DDR2
Flash:    8 MB NOR SPI
Ports:  100 MBit (24V PoE in)
WLAN: 2.4/5 GHz
UART:     1 UART on PCB marked as J1 with 115200 8N1 config
LEDs:       Power, Ethernet, 4x RSSI LEDs (orange, red, 2x green)
Buttons:    Reset

UART connection details

  .---------------------------------.
  |                                 |
[ETH]          J1                 [ANT]
  |    o VCC o RX o TX o GND        |
  `---------------------------------'

Flashing instructions using recovery method over TFTP

 1. Unplug the ethernet cable from the router.
 2. Using paper clip press and hold the router's reset button. Make sure
    you can feel it depressed by the paper clip. Do not release the button
    until step 4.
 3. While keeping the reset button pressed in, plug the ethernet cable
    back into the AP. Keep the reset button depressed until you see the
    device's LEDs flashing in upgrade mode (alternating LED1/LED3 and
    LED2/LED4), this may take up to 25 seconds.
 4. You may release the reset button, now the device should be in TFTP
    transfer mode.
 5. Set a static IP on your Computer's NIC. A static IP of 192.168.1.25/24
    should work.
 6. Plug the PoE injector's LAN cable directly to your computer.
 7. Start tftp client and issue following commands:
     tftp> binary
     tftp> connect 192.168.1.20
     tftp> put openwrt-ar71xx-generic-ubnt-bullet-m-xw-squashfs-factory.bin

Tested only on Bullet M2HP.

Signed-off-by: Petr Štetiar <ynezz@true.cz>