dev-arm: drain implementation for SMMUv3

dev-arm: drain implementation for SMMUv3

SMMUv3 is drained when (1) no SMMU translations are pending
on any of its slave interfaces and (2) no commands are stored
in the Command Queue waiting to be processed.

Change-Id: I81cef5fd821fa5e509e130af02aece5239493df5
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19309
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>