dt-bindings: soc: add MT2712 power dt-bindings

Add power dt-bindings for MT2712.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>