ath79: add support for MikroTik RouterBOARD LHG 2nD

ath79: add support for MikroTik RouterBOARD LHG 2nD

The MikroTik RouterBOARD LHG 2nD (sold as LHG 2) is a 2.4 GHz
802.11b/g/n outdoor device with a feed and an integrated dual
polarization grid dish antenna based on the LHG-HB platform.

See https://mikrotik.com/product/lhg_2 for more info.

Specifications:
 - SoC: Qualcomm Atheros QCA9533
 - RAM: 64 MB
 - Storage: 16 MB NOR
 - Wireless: Atheros AR9531 (SoC) 802.11b/g/n 2x2:2, 18 dBi antenna
 - Ethernet: Atheros AR8229 (SoC), 1x 10/100 port, 12-28 Vdc PoE in
 - 8 user-controllable LEDs:
   · 1x power (blue)
   · 1x user (green)
   · 1x lan (green)
   · 1x wlan (green)
   · 4x rssi (green)

Note:
 The rssihigh LED is disabled, as it shares GPIO 16 with the reset
 button.

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

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[rebase, remove rssiled setup, adjust commit message, add DTSIs]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>