dt-bindings: media: mediatek: mdp3: Add support for MT8188 RDMA

Add a compatible for MediaTek MT8188 RDMA, which supports only a
subset of the MDP3 components of its similar MT8195 counterpart.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>