This implements the "reset controller" side of the clock/reset controller.
It's a simple array of registers and bits to set.
The register table itself comes from Linux; the rest of the code is a
reimplementation.
It doesn't yet implement or expose the clock side - I have a lot of
reverse engineering to do before that!
Reviewed by: andrew, manu, imp
Differential Revision: https://reviews.freebsd.org/D32723
Obtained from: Linux (registers)
