ath79: add support for MikroTik RB951Ui-2HnD

ath79: add support for MikroTik RB951Ui-2HnD

MikroTik RB951Ui-2HnD is a wireless SOHO router that was previously
supported by the ar71xx target, see commit d19b868b12 ("ar71xx: Add
support for MikroTik RB951Ui-2HnD").

Specifications
--------------

  - SoC: Atheros AR9344 (600 MHz)
  - RAM: 128 MB (2x 64 MB)
  - Storage: 128 MB NAND flash (various manufacturers)
  - Ethernet: Atheros AR8229 switch, 5x 10/100 Mbit/s
      - 1x PoE in (port 1, 8-30 V input)
      - 1x PoE out (port 5, 500 mA output)
  - Wireless: Atheros AR9340 (802.11b/g/n)
  - USB: 2.0 (1A)
  - 9x LED:
      - 1x power (green, not configurable)
      - 1x user (green)
      - 5x FE ports (green)
      - 1x wireless (green)
      - 1x PoE out (red)
  - 1x button (restart)

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

Flashing
--------

TFTP boot initramfs image and then perform sysupgrade.  Follow
common MikroTik procedures at https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Michał Kępień <openwrt@kempniu.pl>