iommu/arm-smmu-v3: Forget about cmdq-sync interrupt

The cmdq-sync interrupt is never going to be particularly useful, since
for stage 1 DMA at least we'll often need to wait for sync completion
within someone else's IRQ handler, thus have to implement polling
anyway. Beyond that, the overhead of taking an interrupt, then still
having to grovel around in the queue to figure out *which* sync command
completed, doesn't seem much more attractive than simple polling either.

Furthermore, if an implementation both has wired interrupts and supports
MSIs, then we don't want to be taking the IRQ unnecessarily if we're
using the MSI write to update memory. Let's just make life simpler by
not even bothering to claim it in the first place.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>