ath79: add support for Ubiquiti airCube AC

ath79: add support for Ubiquiti airCube AC

The Ubiquiti Network airCube AC is a cube shaped device supporting
2.4 GHz and 5 GHz with internal 2x2 MIMO antennas.
It can be powered with either one of:
 - 24v power supply with 3.0mm x 1.0mm barrel plug
 - 24v passive PoE on first LAN port
There are four 10/100/1000 Mbps ports (1 * WAN + 3 * LAN).
First LAN port have optional PoE passthrough to the WAN port.

SoC:       Qualcomm / Atheros AR9342
RAM:       64 MB DDR2
Flash:     16 MB SPI NOR
Ethernet:  4x 10/100/1000 Mbps (1 WAN + 3 LAN)
LEDS:      1x via a SPI controller (not yet supported)
Buttons:   1x Reset
Serial:    1x (only RX and TX); 115200 baud, 8N1

Missing features:
 - LED control is not supported

Physical to internal switch port mapping:
 - physical port #1 (poe in) = switchport 2
 - physical port #2 = switchport 3
 - physical port #3 = switchport 5
 - physical port #4 (wan/poe out) = switchport 4

Factory update is tested and is the same as for Ubiquiti AirCube ISP
hence the shared configuration between that devices.

Signed-off-by: Roman Kuzmitskii <damex.pp@icloud.com>