ATSAMD processor family support

[kyp44]

I have implemented and tested RTC-based monotonics (for RTIC v2) for ATSAMD processors, but it is unclear to me where these should reside.

Others are also working on timer-based monotonics.

As the maintainers here have a stake in this, I thought you might be interested in adding to the discussion over at atsamd-rs about this.

I would request that any ongoing discussion happen over there just so it's all in once place. Once we collectively decide where to put this, I can issue a PR to the appropriate project.

[kyp44]

RFC

The RTC-based monotonics are complete and well-tested (at least on SAMx51x platforms), but there are some issues related to possible publication in rtic-monotonics that I am hoping rtic-monotonics maintainers can weight in on:

1. There is a question regarding RTC clock setup. See the description and discussion starting here.
2. There is another question about whether to remove monotonics that are not the best choices for end users. See the description and discussion starting here.