arch-arm: Writes to DCCMVAC shouldn't flush pipeline

arch-arm: Writes to DCCMVAC shouldn't flush pipeline

Writes to DCCMVAC (Data Cache line Clean by VA to PoC) system register
shouldn't flush the pipeline as a result of the operation. This addition
was wrongly introduced for supporting self-modifying code. Software
barriers should be used instead.

Change-Id: Idf0c27d2e49ca01be19888ae5523b8f8eaefa7b3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5362
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>