iommu/arm-smmu: Add a stream map entry iterator

We iterate over the SMEs associated with a master config quite a lot in
various places, and are about to do so even more. Let's wrap the idiom
in a handy iterator macro before the repetition gets out of hand.

Tested-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>