PCI: Add a stub for pci_ats_disabled()

Currently pci_ats_disabled() is only defined when CONFIG_PCI is enabled.
Since we're about to use the function in the Arm SMMUv3 driver, which
could be built with CONFIG_PCI disabled, add a definition of
pci_ats_disabled() for !CONFIG_PCI.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>