soc: qcom: rpmpd: Add parent PD support

In some cases (like with the GPU core clock on GMU-less SoCs) it's
required that we scale more than one voltage domain. This can be achieved
by linking them in a parent-child relationship. Add support for specifying
a parent PD, similarly to what has been done in the RPMhPD driver.

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-v3-8-06a4f448ff90@linaro.org