Support SAM C2x devices

[jboynes]

Summary

Adds support for SAM C20 and C21 devices.

Added the SVD files downloaded from Atmel's site and generated the pac crates using svd2rust. Only modification made to SVD was the removal of some incorrect <enumeratedValue> for fields in GCLK:SYNCBUSY (see atsamc2x.xsl).

Although this is a big commit (sorry) all of the code in pac was generated. Please let me know if there's a better way to submit this.

This is a checkpoint before adding atsamc features to the hal.

Checklist

• CHANGELOG.md for the BSP or HAL updated
• All new or modified code is well documented, especially public items
• No new warnings or clippy suggestions have been introduced (see CI or check locally)

If Adding a new Board

• Board CI added to crates.json
• Board is properly following "Tier 2" conventions, unless otherwise decided to be "Tier 1"

If Adding a new cargo feature to the HAL

• Feature is added to the test matrix for applicable boards / PACs in crates.json

[bradleyharden]

@jboynes, adding the PACs is the right way to start. But let's hold off until we get buy in from some of the other maintainers on #659.

[jboynes]

No worries. I converted this to a draft for now. If it helps, I can split it into two PRs, one with new code and one containing only generated code.

I will create another branch to work on adding a BSP for the atsamc21_xpro that I can use for verification. I think that will help drive the changes needed in hal to support it.