dt-bindings: display: mediatek: merge: Add compatible for MT8188

Add compatible name for MediaTek MT8188 MERGE.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231024130048.14749-4-shawn.sung@mediatek.com/


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