dt-bindings: mediatek: add MT6797 power dt-bindings

This adds power dt-bindings for MT6797

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Kevin-CW Chen <kevin-cw.chen@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>