drivers: rtc: rtc_ll_stm32: add hse clock support

Add support for HSE as the RTC clock source.

The initial implementation has the following limitations:
 - Only whole MHz HSE frequency is supported, e.g. 24.5 MHz will generate a
 compile error.
 - Frequencies equal or above 64 MHz are not supported and will generate a
   compile error.

The implementation has been tested on nucleo_h563zi with:
 - 8 MHz (via hse-bypass)
 - 25 MHz (via external crystal)

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>