drivers: rtc: stm32: handle STM32WB0 series when RTC_ALARM is enabled

RTC alarm is not supported by the driver for now due to the complexity it
would take to work around an hardware erratum. However, it is not possible
to prevent CONFIG_RTC_ALARM from being enabled at Kconfig level.
Modify the RTC driver such that enabling CONFIG_RTC_ALARM on STM32WB0
series does not cause build errors anymore.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>