dt-bindings: display: mediatek: gamma: Add support for MT8188

The gamma LUT setting of MT8188 and MT8195 are the same, so we create
a one of items for MT8188 to reuse the driver data settings of MT8195.

Signed-off-by: Jason-JH.Lin <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: CK Hu <[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,gamma.yaml

···
               - mediatek,mt8192-disp-gamma
               - mediatek,mt8195-disp-gamma
           - const: mediatek,mt8183-disp-gamma
+      - items:
+          - enum:
+              - mediatek,mt8188-disp-gamma
+          - const: mediatek,mt8195-disp-gamma
 
   reg:
     maxItems: 1