Move generic device stuff out of cortex-m

[ahixon]

via rust-embedded/svd2rust#117

Peripheral, and maybe some other structs, aren't really Cortex-M specific. svd2rust currently relies on this crate just for that struct, but if it generates, say, some Rust code for a Cortex-A target instead, it still needs Peripheral but nothing else.

First thought that comes to mind would be to move Peripheral (and friends?) out into a separate crate, and have svd2rust and cortex-m rely on that generic crate instead. But, I'm sure there are many ways to do it.

Thoughts?