ath79: add support for MikroTik RouterBOARD 922UAGS-5HPacD

ath79: add support for MikroTik RouterBOARD 922UAGS-5HPacD

This patch ports support for the MikroTik RouterBOARD 922UAGS-5HPacD
with a built-in 802.11ac High-Power radio (31dBm), which was already
available in the ar71xx target.

See https://mikrotik.com/product/RB922UAGS-5HPacD for more info.

Specifications:
- SoC: Qualcomm Atheros QCA9558 (720 MHz)
- RAM: 128 MB
- Storage: 128 MB NAND
- Wireless: external QCA9882 802.11a/ac 2x2:2
- Ethernet: 1x 1000/100/10 Mbps, integrated, via AR8031 PHY, passive PoE-in 24V
- SFP: 1x host
- USB: 1x 2.0 type A
- PCIe: 1x Mini slot (also contains USB 2.0 for 3G/LTE modems)
- SIM slot: 1x mini-SIM

Working:
- Board/system detection
- SPI and NAND storage
- PCIe
- USB type A host
- Wireless
- Ethernet
- LEDs (user, phy0)
- Reset button
- Sysupgrade to/from ar71xx

Not supported:
- RSSI LEDs
- SFP cage

Installation methods:
- Sysupgrade from ar71xx (it is advisable to use the -n option to
wipe any previous settings), or
- Boot the initramfs image via TFTP and then flash the sysupgrade
image using "sysupgrade -n"

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