ath79: add support for Ubiquiti PowerBridge M

ath79: add support for Ubiquiti PowerBridge M

This adds support for the Ubiquiti PowerBridge M, which has the same
board/LEDs as the Bullet M XM, but different case and antennas.

Specifications:
- AR7241 SoC @ 400 MHz
- 64 MB RAM
- 8 MB SPI flash
- 1x 10/100 Mbps Ethernet, 24 Vdc PoE-in
- Internal antenna: 25 dBi
- POWER/LAN green LEDs
- 4x RSSI LEDs (red, orange, green, green)
- UART (115200 8N1) on PCB

Flashing via WebUI:
Upload the factory image via the stock firmware web UI.

Attention: airOS firmware versions >= 5.6 have a new bootloader with
an incompatible partition table!

Please downgrade to <= 5.5 _before_ flashing OpenWrt!
Refer to the device's Wiki page for further information.

Flashing via TFTP:
Same procedure as other Bullet M (XM) boards.

- 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_powerbridge-m-squashfs-factory.bin

Signed-off-by: Vieno Hakkerinen <vieno@hakkerinen.eu>