example(grand_central_m4): Adding async dmac example

[supersimple33]

Summary

I added the async dmac example from the metro m4.

Checklist

• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced - CI will deny clippy warnings by default! You may #[allow] certain lints where reasonable, but ideally justify those with a short comment.

[ianrrees]

Thanks @supersimple33 ! In #830 we're currently discussing some options to make it a bit easier for us to maintain the BSPs and examples (currently one of the bigger challenges of maintaining this project) - it strikes me that this PR might make a good test case for a new scheme we're considering. Would you be up for trying the proposed approach? I think this would look like:

1. Add my fork as a remote of your local atsamd-rs repo and checkout the manage branch - LMK if you'd like the Git incantations to do that.
2. Change the filename of /examples/metro_m4-async_dmac.rs to /examples/generic-async_dmac.rs.
3. Edit examples/examples.toml to make a new section for the async_dmac example, specifying it's used by both metro_m4 and grand_central_m4.
4. cd in to /manage and do cargo run -- example distribute ../examples ../boards
5. Edit the /boards/grand_central_m4/Cargo.toml just like you've done here (I'd do this via git checkout AsyncDMAC -- boards/grand_central_m4/Cargo.toml)
6. Open a PR against my fork.

Since this is all work-in-progress, there's not much documentation yet and things might be a little rough around the edges. Just curious to get some feedback on what the process looks like (noting that some of this is more involved as the work is in a different fork), and see what snags we encounter.

Also, apologies that this is a bit of a comment-and-run as I'm going to be AFK for a couple days starting shortly.

[supersimple33]

Got it will do

[ianrrees]

Sorry I've dropped the ball on this - we've decided to go with a different approach to deduplicating examples, and I started porting this work to the new scheme here but realised it also means introducing defmt to boards that don't currently use it.

The defmt stuff seems like a separate can of worms; a while back I opened #810 to make it an optional feature of BSPs, so I'm thinking to resolve that using metro_m4 as a test case (partly as I have a Metro M4 to test on), then finally we can use your work in porting the async_dmac examples to metro_m4 and grand_central_m4.