PCI/ATS: Add PASID stubs

The SMMUv3 driver, which may be built without CONFIG_PCI, will soon gain
PASID support.  Partially revert commit c6e9aefb ("PCI/ATS: Remove
unused PRI and PASID stubs") to re-introduce the PASID stubs, and avoid
adding more #ifdefs to the SMMU driver.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>