ath79: add support for Ubiquiti PowerBeam 5AC Gen2

ath79: add support for Ubiquiti PowerBeam 5AC Gen2

The Ubiquiti PowerBeam 5AC Gen 2 (PBE-5AC-Gen2) is an outdoor 802.11ac
5 GHz bridge with a radio feed and a dish antenna. The device is
hardware-compatible with the LiteBeam AC Gen2, plus the 4 extra LEDs.

Specifications:
 - SoC: Qualcomm Atheros AR9342 rev 2
 - RAM: 64 MB DDR2
 - Flash: 16 MB SPI NOR (mx25l12805d)
 - Ethernet: 1x 10/100/1000 Mbps Atheros 8035, 24 Vdc PoE-in
 - WiFi 5 GHz: QCA988x HW2.0 Ubiquiti target 0x4100016c chip_id 0x043222ff
 - WiFi 2.4 GHz: Atheros AR9340 (SoC-based)
 - Buttons: 1x (reset)
 - LEDs: 1x power, 1x Ethernet, 4x RSSI via GPIO. All blue.
 - UART: not tested

Installation from stock airOS firmware:
 - Follow instructions for WA-type Ubiquiti devices on OpenWrt wiki

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[changed device name in commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>