arch-arm: Fault when dc ivac is executed from EL0

arch-arm: Fault when dc ivac is executed from EL0

A previous change enabled execution of dc ivac from EL0 when
SCTLR_EL1.UCI=1. The Arm ARM specifies that dc ivac is the only data
cache maintenance operation by VA that cannot be executed from
EL0. This changeset essential reverts the change:
8d43922 arch-arm: Allow dc ivac from EL0 when SCTLR_EL1.UCI=1

Change-Id: Ia25fab13846a151f548e649a16067feb1ff65c9c
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7823
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>