iommu/amd: Introduce struct amd_iommu_viommu

iommu/amd: Introduce struct amd_iommu_viommu

Which stores reference to nested parent domain assigned during the call to
struct iommu_ops.viommu_init(). Information in the nest parent is needed
when setting up the nested translation.

Note that the viommu initialization will be introduced in subsequent
commit.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>