Add undocumented IOSet

[TDHolmes]

Summary

ATSAMD5x/ATSAME5x has an undocumented IOSet that some of our boards use. Stealing this implementation from @bradleyharden to get it in before the rest of his changes land to un-block #487

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)

[bradleyharden]

@TDHolmes, you only need to add the separate, UndocIoSet type definition to pad.rs. The rest of the changes come from a refactor to reduce the usage of #[cfg(feature = "min-samd51g")]. You can probably leave that refactoring in #467.

[TDHolmes]

Tested this on the feather_m4 branch and it works as expected @bradleyharden . I'll wait for this to merge before touching that branch though