[POWERPC] cell: update Cell BE register definitions

There are a few definitions that are required by subsequent patches,
so add them here.

The original patch is from David Erb, but is significantly cleaned
up by Kevon Corry.

Cc: Kevin Corry <kevcorry@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>