clk: qcom: dispcc-sm8250: handle MMCX power domain

On SM8250 MMCX power domain is required to access MMDS_GDSC registers.
This power domain is expressed as mmcx-supply regulator property. Use
this regulator as MDSS_GDSC supply.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201023131925.334864-6-dmitry.baryshkov@linaro.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>