arch-arm: Use named constants for m5op instructions

arch-arm: Use named constants for m5op instructions

Change-Id: I544519c4f87e50cc02af29cbb3edc31ecf726e8e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4263
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>