PCI: Move ATS declarations outside of CONFIG_PCI

At the moment, the ATS functions are only defined when CONFIG_PCI is
enabled. Since we're about to use them in the Arm SMMUv3 driver, which
could be built with CONFIG_PCI disabled, and they are already guarded by
CONFIG_PCI_ATS which depends on CONFIG_PCI, move the definitions outside
of 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>