drivers: rtc: stm32: avoid using LL_RTC_Init

LL_RTC_Init() internally uses LL_RTC_EnterInitMode() wich uses a blocking
while loop. This rewrite uses the less CPU time wasteful local
rtc_stm32_enter_initialization_mode() instead. As a side effect
LL_RTC_InitTypeDef in rtc_stm32_config is not needed anymore.

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