AMD IOMMU: save pci segment from ACPI tables

This patch adds the pci_seg field to the amd_iommu structure and fills
it with the corresponding value from the ACPI table.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>