dt-bindings: display: mediatek: dp: Add compatible for MediaTek MT8188

Add dt-binding documentation of dp-tx for MediaTek MT8188 SoC.

Signed-off-by: Shuijing Li <[e-mail]>
Signed-off-by: Jitao Shi <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[e-mail]/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

Documentation/devicetree/bindings/display/mediatek/mediatek,dp.yaml

···
 properties:
   compatible:
     enum:
+      - mediatek,mt8188-dp-tx
+      - mediatek,mt8188-edp-tx
       - mediatek,mt8195-dp-tx
       - mediatek,mt8195-edp-tx