Add a way for firmware to tell the OS that ATS is supported by the PCI
root complex. An endpoint with ATS enabled may send Translation Requests
and Translated Memory Requests, which look just like Normal Memory
Requests with a non-zero AT field. So a root controller that ignores the
AT field may simply forward the request to the IOMMU as a Normal Memory
Request, which could end badly. In any case, the endpoint will be
unusable.

The ats-supported property allows the OS to only enable ATS in endpoints
if the root controller can handle ATS requests. Only add the property to
pcie-host-ecam-generic for the moment. For non-generic root controllers,
availability of ATS can be inferred from the compatible string.

Link: https://lore.kernel.org/linux-pci/20240607105415.2501934-3-jean-philippe@linaro.orgSigned-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>