[POWERPC] Add briq support to CHRP

The support for Briq machines has been floating around as patches for
ages. This cleans it up and adds it once for all.

Some of this is based on initial code provided by Karsten Jeppesen
<karsten@jeppesens.com> and mostly rewritten from scratch by me.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>