ar71xx: add kernel support for the Mikrotik RB951G board

ar71xx: add kernel support for the Mikrotik RB951G board

This is platform definition for the RB951G device. It's mostly rework
of RB2011UAS patchset with network taken from the RB751 patches.
The main difference is the flash layout used by this device.

[juhosg:
  - remove dead code from mach-rb951g
  - add "-2HnD" suffix to the machine name
  - use a separate 615-MIPS-ath79-RB951G-support.patch]

Patchwork: http://patchwork.openwrt.org/patch/3257/
Signed-off-by: Kamil Trzcinski <ayufan@ayufan.eu>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35633 3c298f89-4303-0410-b956-a3cf2f4a3e73