iommu/arm-smmu-v3: Implement flush_iotlb_all hook

.flush_iotlb_all is currently stubbed to arm_smmu_iotlb_sync() since the
only time it would ever need to actually do anything is for callers
doing their own explicit batching, e.g.:

	iommu_unmap_fast(domain, ...);
	iommu_unmap_fast(domain, ...);
	iommu_iotlb_flush_all(domain, ...);

where since io-pgtable still issues the TLBI commands implicitly in the
unmap instead of implementing .iotlb_range_add, the "flush" only needs
to ensure completion of those already-in-flight invalidations.

However, we're about to start using it in anger with flush queues, so
let's get a proper implementation wired up.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
[rm: document why it wasn't a bug]
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>