arch-arm: Unify permission checks for dc * instructions

Change-Id: Ib47f4134e3f0a580e5356d384a5d3b293c1af7be
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7828
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>