iommu/arm-smmu-v3: Store SteamIDs in master

Simplify the attach/detach code a bit by keeping a pointer to the stream
IDs in the master structure. Although not completely obvious here, it does
make the subsequent support for ATS, PRI and PASID a bit simpler.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>