Do not DerefMut into PAC register blocks

[bugadani]

This PR is a part of #2661 although that issue needs more work. In the mean time, this PR removes the virtualness (which isn't generally true) from peripheral singleton documentation, removes remnants of an old macro syntax, and removes the DerefMut impl that isn't needed.

Follow-up will include:

• Removing logic from peripheral singletons and traits implemented for peripheral singletons
• A review of GPIO interconnect's types - which ones are technically peripheral singletons in this context?
• Use &self in the remaining peripheral singleton methods (in the spirit of GPIO: Avoiding &mut self when &self is safe to use. #2711 but not quite)
• Removing DerefMut from PeripheralRef
• Most likely removing the blanket Sealed for &mut impl Sealed implementation

[bjoernQ]

LGTM

[JurajSadel]

Thanks!