iommu/arm-smmu-v3: Add a master->domain pointer

As we're going to track domain-master links more closely for ATS and CD
invalidation, add pointer to the attached domain in struct
arm_smmu_master. As a result, arm_smmu_strtab_ent is redundant and can be
removed.

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