arm64: dts: mediatek: mt8173: Add power domain to encoder nodes

The power of encoder is not control by mediatek,larb, so we add
power domain to encoder nodes for mt8173 SoC.

Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
Link: https://lore.kernel.org/r/20220421035111.7267-4-allen-kh.cheng@mediatek.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>