fixed(rtc): Fixes problems with the rtc::Rtc abstraction

feat(rtc): Fixes issues with the `rtc::Rtc` abstraction, and updates it to utilize the `modes` modules abstractions used by the RTIC RTC monotonics (#845).

* Adds implementation of `embedded_hal::delay::DelayNs` to `Rtc<Count32Mode>`.
* Removes private methods of `Rtc` that are no longer needed.
* Reworks `rtc::TimerParams` a bit to better detect unreachable required prescaler values.
* Adds RTC mode 2 (i.e. clock mode) support in the private `rtc::modes` module.

---------

Co-authored-by: Dan Whitman <[email protected]>

Author: kyp44