arm: Replace MiscReg with RegVal in utility.(hh|cc).

arm: Replace MiscReg with RegVal in utility.(hh|cc).

These uses snuck in after the previous pass which made this switch in
the rest of these files.

Change-Id: Ie891c6ec393a65f1c57c54301f0a2bb920d38bb0
Reviewed-on: https://gem5-review.googlesource.com/c/15795
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>