Enhancement: Restore all registers to fatcory default

[gr-butler]

As you may have seen in various forums, there seems to be an issue with these devices suddenly failing to respond. I've updated to your library and the self-test passes, but the reader does not respond to tags - the issue a lot of people have.

Memory/stack corruption seems to pop up from time to time in possible explanations and my gut feeling is that maybe the MCRF522 registers have been corrupted. So I've love to try a "reset-to-factory" operation to see if this is the case.

This hopefully would also be a generally useful function.