soc: mediatek: Add MT2701 power dt-bindings

Add power dt-bindings for MT2701.

Signed-off-by: Shunli Wang <shunli.wang@mediatek.com>
Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>