Remove Started/Stopped type state from Rtc

[timokroeger]

The type states make it difficult to enable and disable the RTC on demand with RTIC.
Started/Stopped types were only used to restrict the usage of set_prescaler() to a disabled RTC.
Pass the prescaler as argument to the costructor instead.

[jonas-schievink]

Thanks, this looks reasonable!

This does mean that you can't change the prescaler after creating the RTC though. I don't think that's a common thing to do, so it should be fine, but we might want to add a fn free(self) -> RTC to enable that use case again.

[jonas-schievink]

Oh, it already has a release function that does exactly that.