iommu/amd: Add iommuv2 flag to struct amd_iommu

In mixed IOMMU setups this flag inidicates whether an IOMMU
supports the v2 features or not. This patch also adds a
global flag together with a function to query that flag from
other code. The flag shows if at least one IOMMUv2 is in the
system.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>