arm: dts: mediatek: converted to using SPDX identifiers

Convert boilerplate license statement into proper SPDX identifier style.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>