iommu/arm-smmu: put iommu_domain pointer in dev->archdata.iommu

In preparation for nested translation support, stick a pointer to the
iommu_domain in dev->archdata.iommu. This makes it much easier to grab
hold of the physical group configuration (e.g. cbndx) when dealing with
vSMMU accesses from a guest.

Signed-off-by: Will Deacon <will.deacon@arm.com>