media: dt-bindings: mtk-vcodec-encoder: Add power-domains property

The encoder node may be dependent on a power-domain. Add a property for
it.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>