In preparation for allowing the default domain type to be overridden,
this patch adds support for IOMMU_DOMAIN_IDENTITY domains to the
ARM SMMUv3 driver.

An identity domain is created by placing the corresponding stream table
entries into "bypass" mode, which allows transactions to flow through
the SMMU without any translation.
Signed-off-by: Will Deacon <will.deacon@arm.com>