arm64: dts: qcom: sc8280xp-pmics: add pmk8280 rtc

The PMK8280 has an RTC which can also be used as a wakeup source.

Note that the RTC can not be disabled and updating the time is not
permitted either. Instead an offset can be stored in some other machine-
specific non-volatile memory which a driver can take into account.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230202155448.6715-20-johan+linaro@kernel.org