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/

Documentation/devicetree/bindings/mips/ralink.txt

+Ralink MIPS SoC device tree bindings
+
+1. SoCs
+
+Each device tree must specify a compatible value for the Ralink SoC
+it uses in the compatible property of the root node. The compatible
+value must be one of the following values:
+
+  ralink,rt2880-soc
+  ralink,rt3050-soc
+  ralink,rt3052-soc
+  ralink,rt3350-soc
+  ralink,rt3352-soc
+  ralink,rt3883-soc
+  ralink,rt5350-soc
+  ralink,mt7620a-soc
+  ralink,mt7620n-soc