mips: Stop using architecture specific register types.

mips: Stop using architecture specific register types.

Change-Id: I764f6eea214ba4e03cc0fe19a21abcb0ebd04408
Reviewed-on: https://gem5-review.googlesource.com/c/14462
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>