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.