ath79: add support for Mikrotik LHG 5

ath79: add support for Mikrotik LHG 5

The MikroTik LHG 5 series (product codes RBLHG-5nD, RBLHG-5HPnD and
RBLHG-5HPnD-XL) devices are an outdoor 5GHz CPE with a 24.5dBi or 27dBi
integrated antenna built around the Atheros AR9344 SoC.
It is very similar to the SXT Lite5 series which this patch is based
upon.

Specifications:
 - SoC: Atheros AR9344
 - RAM: 64 MB
 - Storage: 16 MB SPI NOR
 - Wireless: Atheros AR9340 (SoC) 802.11a/n 2x2:2
 - Ethernet: Atheros AR8229 switch (SoC), 1x 10/100 port,
    8-32 Vdc PoE in
 - 8 user-controllable LEDs:
  - 1x power (blue)
  - 1x user (white)
  - 1x ethernet (green)
  - 5x rssi (green)

 See https://mikrotik.com/product/RBLHG-5nD for more details.

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

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

Signed-off-by: Jakob Riepler <jakob+openwrt@chaosfield.at>