arm64: dts: qcom: sc8280xp: add rpmh-stats node

Add a node describing the RPMh shared memory that can be used to
retrieve statistics for the SoC low-power modes.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220921080050.21383-1-johan+linaro@kernel.org