PCI/ATS: Restore EXPORT_SYMBOL_GPL() for pci_{enable,disable}_ats()

Commit d355bb20

 ("PCI/ATS: Remove unnecessary EXPORT_SYMBOL_GPL()")
unexported a bunch of symbols from the PCI core since the only external
users were non-modular IOMMU drivers. Although most of those symbols
can remain private for now, 'pci_{enable,disable_ats()' is required for
the ARM SMMUv3 driver to build as a module, otherwise we get a build
failure as follows:

  | ERROR: "pci_enable_ats" [drivers/iommu/arm-smmu-v3.ko] undefined!
  | ERROR: "pci_disable_ats" [drivers/iommu/arm-smmu-v3.ko] undefined!

Re-export these two functions so that the ARM SMMUv3 driver can be build
as a module.

Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
[will: rewrote commit message]
Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: John Garry <john.garry@huawei.com> # smmu v3
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>