Import Mediatek/Ralink dtsi patches against OpenWRT dtsi files

[sgalabov]

Summary

This revision suggests dtsi patches to be used with the original OpenWRT dtsi files so we can re-use what has already been done in OpenWRT for the Mediatek/Ralink SoCs.

The only thing that is required after importing this revision should be the following:

1. Import OpenWRT dts/dtsi files into sys/gnu/dts/mips ( from, e.g., https://git.openwrt.org/?p=openwrt.git;a=tree;f=target/linux/ramips/dts;h=fee448930238ce812cf5f4c023ae3831b322114f;hb=HEAD )
2. Run the following script in sys/gnu/dts/mips:

for f in `ls [mr]t*.dtsi`; do
printf "\n#include <fbsd-$f>\n" > $f
done

This will apply our dtsi patches to OpenWRT's dtsi files and will allow us to re-use dts/dtsi files for ~170 Mediatek/Ralink boards.

Currently our drivers are not 100% compatible with OpenWRT's dts files, but they're compatible enough.
We can add more functionality in the future that would better leverage the OpenWRT work as well.