Add SAMD20J18A support

[kaizensparc]

Summary

This Pull Request integrates the SAMD20J18A microcontroller:

• It adds the SVD file and its XSL patches in the svd directory
• It generates the corresponding pac
• It updates the HAL to have support for it
• It adds the SAMD20 Xplained Pro board in the BSP with a bliny_basic example

This is only a proof of concept, not all features have been tested in real life even though everything compiles fine :)

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

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

[Sympatron]

I'm not that familiar with SAMD20. But I think it's mostly SAMD21 without USB. So it's probably useful to have a common feature for them (samd2x?) to simplify the #[cfg]s.

[kaizensparc]

I'm not that familiar with SAMD20. But I think it's mostly SAMD21 without USB. So it's probably useful to have a common feature for them (samd2x?) to simplify the #[cfg]s.

Yes, good idea! I'll try to make a samd2x feature which is activated from any of the samd20/samd21 features. I never wrote anything with features yet so that's going to be something new to learn :)