drivers: rtc: stm32: remove calls from ll source file

Remove all function calls for functions that are defined in the
stm32xxxx_ll_rtc.c and replaces them by simpler calls from the header file.
Also remove the use of LL_RTC_AlarmTypeDef since no functions from the
header file uses it.

Signed-off-by: Guillaume Gautier <[email protected]>

Author: gautierg-st