x86/amd-iommu: Add an index field to struct amd_iommu

This patch adds an index field to struct amd_iommu which can
be used to lookup it up in an array. This index will be used
in struct protection_domain to keep track which protection
domain has devices behind which IOMMU.

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