ath79: add support for MikroTik hAP (RB951Ui-2nD)

ath79: add support for MikroTik hAP (RB951Ui-2nD)

The MikroTik hAP (product code RB951Ui-2nD) is
an indoor 2.4Ghz AP with a 2 dBi integrated antenna built around the
Atheros QCA9531 SoC.

Specifications:
 - SoC: Atheros QCA9531
 - RAM: 64 MB
 - Storage: 16 MB NOR - Winbond 25Q128FVSG
 - Wireless: Atheros QCA9530 (SoC) 802.11b/g/n 2x2
 - Ethernet: Atheros AR934X switch, 5x 10/100 ports,
   10-28 V passive PoE in port 1, 500 mA PoE out on port 5
 - 8 user-controllable LEDs:
   · 1x power (green)
   · 1x user (green)
   · 4x LAN status (green)
   · 1x WAN status (green)
   · 1x PoE power status (red)

See https://mikrotik.com/product/RB951Ui-2nD for more details.

Notes:
 The device was already supported in the ar71xx target.

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. Follow common
 MikroTik procedure as in https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Maciej Krüger <mkg20001@gmail.com>