iommu/arm-smmu-v3: Specialise CMD_SYNC handling

CMD_SYNC already has a bit of special treatment here and there, but as
we're about to extend it with more functionality for completing outside
the CMDQ lock, things are going to get rather messy if we keep trying to
cram everything into a single generic command interface. Instead, let's
break out the issuing of CMD_SYNC into its own specific helper where
upcoming changes will have room to breathe.

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