dt-bindings: display/msm/gmu: Document Adreno 750 GMU

Document the Adreno 750 GMU found on the SM8650 platform.
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/578684/Signed-off-by: Rob Clark <robdclark@chromium.org>

Documentation/devicetree/bindings/display/msm/gmu.yaml

@@ -224,6 +224,7 @@ allOf:
             enum:
               - qcom,adreno-gmu-730.1
               - qcom,adreno-gmu-740.1
+              - qcom,adreno-gmu-750.1
     then:
       properties:
         reg: