dt-bindings: soc: qcom: Update devicetree binding document for rpmh-rsc

The change documents power-domains property for RSC device.
This optional property points to corresponding PM domain node.

Signed-off-by: Maulik Shah <quic_mkshah@quicinc.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> # SM8450
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221018152837.619426-2-ulf.hansson@linaro.org