ath79: add support for MikroTik RB951G-2HnD

ath79: add support for MikroTik RB951G-2HnD

MikroTik RB951G-2HnD is a wireless SOHO router that was previously
supported by the ar71xx target, see commit 7a709573d7 ("ar71xx: add
kernel support for the Mikrotik RB951G board").

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

  - SoC: Atheros AR9344 (600 MHz)
  - RAM: 128 MB (2x 64 MB)
  - Storage: 128 MB NAND flash (various manufacturers)
  - Ethernet: Atheros AR8327 switch, 5x 10/100/1000 Mbit/s
      - 1x PoE in (port 1, 8-30 V input)
  - Wireless: Atheros AR9340 (802.11b/g/n)
  - USB: 2.0 (1A)
  - 8x LED:
      - 1x power (green, not configurable)
      - 1x user (green, not configurable)
      - 5x GE ports (green, not configurable)
      - 1x wireless (green, not configurable)
  - 1x button (restart)

Unlike on the RB951Ui-2HnD, none of the LEDs on this device seem to be
GPIO-controllable, which was also the case for older OpenWRT versions
that supported this board via a mach file.  The Ethernet port LEDs are
controlled by the switch chip.

See https://mikrotik.com/product/RB951G-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>
(cherry picked from commit db02cecd6ad2e5962e6e9d8307da34855a083ad6)