drivers: rtc: stm32: allow new RTC driver to work with BBRAM

STM32 BBRAM depends on RTC to work. This changes STM32 RTC init stage to
PRE_KERNEL_1 to allow RTC driver to initialize before BBRAM driver.

Some adjustments are made so that kernel API is not used during the init
procedure.

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