iommu/arm-smmu: Install bypass S2CRs for IOMMU_DOMAIN_IDENTITY domains

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

An identity domain is created by placing the corresponding S2CR
registers into "bypass" mode, which allows transactions to flow through
the SMMU without any translation.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>