Move STM32 RTC to new counter API

[erwango]

Is your feature request related to a problem? Please describe.
Move STM32 RTC driver to new counter API (cf #8340)

Describe the solution you'd like
Could look at #11347 as example of similar work

[galak]

@ydamigos Is this something you might have some time to look into doing?

[ydamigos]

@galak My time is limited but I'll give it a try,

[erwango]

@ydamigos , have you started? I should find some time to check it this week.

[ydamigos]

@erwango yes, I did some work on it. You could find it here. It compiles but it doesn't work on target.

[erwango]

ok, I finally started as well :-). It is here. Not linking with Cube for some issue for now.
I took some options that could be discussed since new API doesn't fit nicely STM32 RTC IP, but out of our 2 initiatives we should manage to get through.