dt-bindings: clock: qcom,dispcc-sm8x50: add mmcx power domain

On sm8250 dispcc requires MMCX power domain to be powered up before
clock controller's registers become available. For now sm8250 was using
external regulator driven by the power domain to describe this
relationship. Switch into specifying power-domain and required opp-state
directly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>