soc: qcom: rpmpd: Hook up VDDMX as parent of SM6375 VDDGX

The GPU core clock requires that both VDDGX and VDDMX domains are scaled
at the same rate at the same time (well, MX just before GX but you get
the idea). Set MX as parent of GX to take care of that.

Suggested-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230313-topic-rpmpd-v3-9-06a4f448ff90@linaro.org