dt-bindings: display: mediatek: dpi: add power-domains example

DPI is part of the display / multimedia block in MediaTek SoCs, and
always have a power-domain (at least in the upstream device-trees).
Add the power-domains property to the binding example.

Fixes: 9273cf7d ("dt-bindings: display: mediatek: convert the dpi bindings to yaml")
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231023-display-support-v7-1-6703f3e26831@baylibre.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>