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

* 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 prescalar values.
* Adds RTC mode 2 (i.e. clock mode) support in the private `rtc::modes` module.

Author: kyp44