Add RTCv3 for the STM32C0

The STM32C0 supports v3 of RTC. However it only supports a subset of the features.

Notably the differences are:
* No Wakeup unit / No Wakeup event
* Alarm is referenced as AlarmA

References:
* STM32 RTC v2/v3: AN4759 - Rev 10: https://www.st.com/resource/en/application_note/an4759-introduction-to-using-the-hardware-realtime-clock-rtc-and-the-tamper-management-unit-tamp-with-stm32-mcus-stmicroelectronics.pdf
* STM32C0: RM0490 - Rev 5: https://www.st.com/resource/en/reference_manual/rm0490-stm32c0x1-advanced-armbased-32bit-mcus-stmicroelectronics.pdf

Tested on STM32C011F6P6

Author: AlanRosenthal