media: dt-bindings: mediatek: Add phandle to mediatek,scp on MDP3 RDMA

The MDP3 RDMA needs to communicate with the SCP remote processor: allow
specifying a phandle to a SCP core.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>