arm: Add support for the dc {civac, cvac, cvau, ivac} instr

arm: Add support for the dc {civac, cvac, cvau, ivac} instr

This patch adds support for decoding and executing the following ARMv8
cache maintenance instructions by Virtual Address:
* dc civac: Clean and Invalidate by Virtual Address to the Point
            of Coherency
* dc cvac: Clean by Virtual Address to the Point of Coherency
* dc cvau: Clean by Virtual Address to the Point of Unification
* dc ivac: Invalidate by Virtual Addrsess to the Point of Coherency

Change-Id: I58cabda37f9636105fda1b1e84a0a04965fb5670
Reviewed-by: Sudhanshu Jha <sudhanshu.jha@arm.com>
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5060
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>