PCI: Export ATS functions to modules

This patch makes the ATS functions usable for modules.
They will be used by a module implementing some advanced
AMD IOMMU features.

Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>