PCI/ATS: Add PASID stubs

The SMMUv3 driver, which may be built without CONFIG_PCI, will soon gain
PASID support.  Partially revert commit c6e9aef ("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>

include/linux/pci-ats.h

@@ -33,6 +33,9 @@ void pci_disable_pasid(struct pci_dev *pdev);
 int pci_pasid_features(struct pci_dev *pdev);
 int pci_max_pasids(struct pci_dev *pdev);
 #else /* CONFIG_PCI_PASID */
+static inline int pci_enable_pasid(struct pci_dev *pdev, int features)
+{ return -EINVAL; }
+static inline void pci_disable_pasid(struct pci_dev *pdev) { }
 static inline int pci_pasid_features(struct pci_dev *pdev)
 { return -EINVAL; }
 static inline int pci_max_pasids(struct pci_dev *pdev)