DT: add documentation for the Ralink MIPS SoCs

This patch adds binding documentation for the
compatible values of the Ralink MIPS SoCs.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: http://patchwork.linux-mips.org/patch/5187/