x86, AMD IOMMU: add functions to find last possible PCI device for IOMMU

This patch adds functions to find the last PCI bus/device/function the IOMMU
driver has to handle. This information is used later to allocate the memory for
the data structures.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: Sebastian.Biemueller@amd.com
Cc: robert.richter@amd.com
Cc: joro@8bytes.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>