PCI: add ACS validation utility

In a PCI environment, transactions aren't always required to reach
the root bus before being re-routed.  Intermediate switches between
an endpoint and the root bus can redirect DMA back downstream before
things like IOMMUs have a chance to intervene.  Legacy PCI is always
susceptible to this as it operates on a shared bus.  PCIe added a
new capability to describe and control this behavior, Access Control
Services, or ACS.

The utility function pci_acs_enabled() allows us to test the ACS
capabilities of an individual devices against a set of flags while
pci_acs_path_enabled() tests a complete path from a given downstream
device up to the specified upstream device.  We also include the
ability to add device specific tests as it's likely we'll see
devices that do not implement ACS, but want to indicate support
for various capabilities in this space.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>