DT: MIPS: ralink: add MT7620A dts files

Add a dtsi file for MT7620A SoC and a sample dts file.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: http://patchwork.linux-mips.org/patch/5190/