Import Mediatek/Ralink dts files from OpenWRT

Import original OpenWRT dts files after executing the following script
on them:

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

Approved by: adrian (mentor)
Obtained from: OpenWRT
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5971