drivers: rtc: stm32: ensure thread safe operations

This adds mutex a lock in rtc_stm32_set_time and rtc_stm32_get_time to
ensure consistent data reading. Also performs register reading in a single
operation in rtc_stm32_get_calibration for the same reason.

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>