arch-arm: Fix printing of the data cache maintenance instructions

Change-Id: I2322c7bf65b38cb07a1ea2b5dc25dfc5a0496cf0
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7825
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>