Make RCC enr() and rstr() public

[burrbull]

Functions for enable, reset peripherals can be used in other crates or in main.

[japaric]

Thanks for the PR, @burrbull.

I'd rather not make these public as they would let you break all the other abstractions. For example, you create / initialize an instance of Serial but a*b.rstr() lets you reset peripherals and undo the Serial configuration.

Is there a specific use case that you need these changes for? Perhaps there's some other way to achieve it.