arch-arm: Change the type of fault for dc ivac instructions

Change-Id: I00f957a3bc4721a66db62b1257f10e9019a94608
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7829
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>