Import Mediatek/Ralink dtsi patches against OpenWRT dtsi files

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
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.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5965