arm: Remove ignored const qualifier

arm: Remove ignored const qualifier

gcc8 warns about ignored const qualifiers (-Wignored-qualifiers) and
that breaks builds.  It was suggested that the warning be moved to
Wextra[1] but that's probably not going to happen anytime soon.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82711

Change-Id: Ib808906deb9a1c2dccb1c34b6563db0c24c66655
Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/8562
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>