arch-arm: Dsb instruction shouldn't flush the pipeline

arch-arm: Dsb instruction shouldn't flush the pipeline

DSB Instruction shouldn't flush the pipeline, hence the IsSquashAfter
attribute will be removed for either the 32 and 64 bit version.

Change-Id: I98b2b8bc78aa28445ed1a9b5f34645f8d71616ad
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5363
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>