ath79: add support for MikroTik RouterBOARD wAP G-5HacT2HnD

ath79: add support for MikroTik RouterBOARD wAP G-5HacT2HnD

This commit 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 port.

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 and LED
 - Reset button

Not working/Unsupported:
 - 2.4 GHz LED
 - AP/CAP LED
 - ZT2046Q SPI temperature and voltage sensor

This adds the basic features for supporting MikroTik devices:
 - a common recipe for mikrotik images in common-mikrotik.mk
 - support for minor (MikroTik NOR) split firmware (only for
   generic subtarget so far)

Acknowledgments: Robert Marko <robimarko@gmail.com>
                 Andrew Cameron <apcameron@softhome.net>
                 Koen Vandeputte <koen.vandeputte@ncentric.com>
                 Chuanhong Guo <gch981213@gmail.com>

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Co-developed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>