ar71xx: add support for MikroTik RouterBOARD wAP G-5HacT2HnD (wAP AC)

ar71xx: add support for MikroTik RouterBOARD wAP G-5HacT2HnD (wAP AC)

This patch adds support for the MikroTik RouterBOARD wAP G-5HacT2HnD (wAP
AC), a small weatherproof dual band, dual-radio 802.11ac wireless AP with
integrated omnidirectional anntennae and one 10/100/1000 Mbps Ethernet
ports.

See https://mikrotik.com/product/RBwAPG-5HacT2HnD for more info.

Specifications:
 - SoC: Qualcomm Atheros QCA9556
 - RAM: 64 MB
 - Storage: 16 MB NOR
 - Wireless:
   · Atheros AR9550 (SoC) 802.11b/g/n 2x2:2, 2 dBi antennae
   · Qualcomm QCA9880 802.11a/n/ac 3x3:3, 2 dBi antennae
 - Ethernet: Atheros AG71xx (SoC, AR8033), 1x 1000/100/10 port, passive
   PoE in

Working:
 - Board/system detection
 - Sysupgrade
 - Serial console
 - Ethernet
 - 2.4 GHz radio
 - 5 GHz radio
 - Reset button

Not working:
 - LEDs (added according to Mikrotik's GPL sources but not functional)

 Unsupported:
 - ZT2046Q SPI temperature and voltage sensor

Contributors: Giuseppe Tipaldi (@Ciusss89)
              Ricky (@rickydee)

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[daniel@makrotopia.org: whitespace fix, use PHYADDR instead of PHYMASK]