power: Get rid of some ISA specific register types.

Change-Id: If63acb10705a9f442255680917d16630748ca8e1
Reviewed-on: https://gem5-review.googlesource.com/c/14465
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>