ath79: Add support for Ubiquity Bullet M (XW)

ath79: 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

 A) Serial console, U-Boot and TFTP

   1. Connect to serial header J1 on the PCB
   2. Power on device and enter U-Boot console
   3. Set up TFTP server serving an OpenWrt initramfs build
   4. Load initramfs build using the command tftpboot in the U-Boot cli
   5. Boot the loaded image using the command bootm
   6. Copy squashfs OpenWrt sysupgrade build to the booted device
   7. Use mtd to write sysupgrade to partition "firmware"
   8. Reboot and enjoy

 B) Experimental factory image flashing over SSH from airOS v6.1.7

   1. You need to flash your UBNT M2HP with airOS v6.1.7 firmware
      no other airOS version is currently supported
   2. git clone https://github.com/true-systems/ubnt-bullet-m2hp-openwrt-flashing
   3. cd ubnt-bullet-m2hp-openwrt-flashing
   4. make flash-factory FW_OWRT=/path/to/your/openwrt-ath79-generic-ubnt_bullet-m-xw-squashfs-factory.bin

Tested only on Bullet M2HP.

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