mips: Change the integer and fp register widths to be 64 bits.

mips: Change the integer and fp register widths to be 64 bits.

This makes them more consistent with the other architectures, helping
to facilitate having a single, unified type across ISAs in the future.

Change-Id: I65efb10e39d453281b8429eeedbb46fa6b023a2b
Reviewed-on: https://gem5-review.googlesource.com/c/13620
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>