Properly trim the vendor tree to include only those files that we want

Properly trim the vendor tree to include only those files that we want
merged into FreeBSD. Cherry picking from a full vendor tree was too
hard and lead to undestirable svn results.

Note: We only tim the dts* files, we don't trim the dt-bindings tree,
since having all of them causes no problems and the benefit to
trimming there is far out weighed by the cost of doing the trim each
time.