dt-bindings: soc: qcom: smd-rpm: add PMIC regulators nodes

The Qualcomm RPM over SMD contains devices for one or two PMIC
regulators - already used in several DTS files.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220926092104.111449-1-krzysztof.kozlowski@linaro.org