MIPS: ralink: adds OF code

Until there is a generic MIPS way of handing the DTB over from bootloader to
kernel we rely on a built in devicetrees. The OF code also remaps those register
ranges that we use global in our drivers.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4895/