drm/mediatek: Add MERGE support for mediatek-drm

Add MERGE engine file:
MERGE module is used to merge two slice-per-line inputs
into one side-by-side output.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220526102126.19756-3-jason-jh.lin@mediatek.com/
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Acked-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>