dt-bindings: display/msm/gmu: Allow passing QMP handle

When booting the GMU, the QMP mailbox should be pinged about some tunables
(e.g. adaptive clock distribution state). To achieve that, a reference to
it is necessary. Allow it and require it with A730.

Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> # sm8450
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/559279/


Signed-off-by: Rob Clark <robdclark@chromium.org>