dt-bindings: dsi-controller-main: Document missing msm8976 compatible

When all dsi-ctrl compats were added msm8976 was missed, include it too.

Signed-off-by: Adam Skladowski <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/575288/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Baryshkov <[e-mail]>

Documentation/devicetree/bindings/display/msm/dsi-controller-main.yaml

···
               - qcom,msm8916-dsi-ctrl
               - qcom,msm8953-dsi-ctrl
               - qcom,msm8974-dsi-ctrl
+              - qcom,msm8976-dsi-ctrl
               - qcom,msm8996-dsi-ctrl
               - qcom,msm8998-dsi-ctrl
               - qcom,qcm2290-dsi-ctrl
···
           contains:
             enum:
               - qcom,msm8953-dsi-ctrl
+              - qcom,msm8976-dsi-ctrl
     then:
       properties:
         clocks: