ar71xx: add initial support for the Mikrotik RB911G/RB912UAG boards

It is only on RB911G-5HPnD and RB912UAG-5HPnD boards.
The LEDs and the USB port is not working yet.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39102 3c298f89-4303-0410-b956-a3cf2f4a3e73