ar71xx: add support for MikroTik RouterBOARD 921GS-5HPacD r2 (mANTBox 15s)

This patch adds support for the MikroTik RouterBOARD 921GS-5HPacD r2
(mANTBox 15s), an outdoor sector antenna with a built-in 802.11ac
wireless router. Additionally, it adds a new profile for devices with
>= 128 MB NAND flash and 802.11ac to the ar71xx/mikrotik subtarget.

See https://mikrotik.com/product/RB921GS-5HPacD-15S for more info.

Specifications:
 - SoC: Qualcomm Atheros QCA9558 (720 MHz)
 - RAM: 128 MB
 - Storage: 128 MB NAND
 - Wireless: external QCA9822 802.11a/ac 2x2:2
 - Ethernet: 1x 1000/100/10 Mbps, integrated, via AR8031 PHY, passive PoE in
 - SFP: 1x host

Working:
 - Board/system detection
 - NAND storage detection
 - Wireless
 - Ethernet
 - 1x user LED
 - Reset button
 - Sysupgrade

Untested:
 - SFP cage (probably not working)

Installation:
 - Boot initramfs image via TFTP and then flash sysupgrade image

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>