Implement a driver for the PowerPC-base RouterBoard (RB333/600/800/1100)

Summary:
This has been tested on the RB800, but should work on the RB333, RB600, and
RB1100 as well.

It's currently missing ECC support, but read and write are complete.

Reviewers: imp

Reviewed By: imp

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D2223