arm64: dts: mt2712: add pintcrl file

This patch adds pinctrl file for mt2712.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>