ipq4018: add initial reset driver support for the clock/reset controller.

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)