PCI: Move pci_ari_enabled() to global header

pci_ari_enabled() is useful outside of drivers/pci, particularly for
deriving INTx routing via ACPI _PRT, so move it to the global header.
Also convert to bool return.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Don Dutile <ddutile@redhat.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>