iommu/vt-d: Extend dmar_domain to support nested domain

The nested domain fields are exclusive to those that used for a DMA
remapping domain. Use union to avoid memory waste.

Link: https://lore.kernel.org/r/20231026044216.64964-3-yi.l.liu@intel.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>