media: platform: mtk-mdp3: Use devicetree phandle to retrieve SCP

Instead of walking the entire parent node for something that has the
right compatible, use the scp_get() function provided by the MediaTek
SCP remoteproc driver to retrieve a handle to mtk_scp through the
devicetree "mediatek,scp" (phandle) property.

In case of multi-core SCP, this also allows to select a specific core.

Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>