dt-bindings: document PASID property for IOMMU masters

On Arm systems, some platform devices behind an SMMU may support the PASID
feature, which offers multiple address space. Let the firmware tell us
when a device supports PASID.
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Reviewed-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: NWill Deacon <will@kernel.org>

Documentation/devicetree/bindings/iommu/iommu.txt

@@ -86,6 +86,12 @@ have a means to turn off translation. But it is invalid in such cases to
 disable the IOMMU's device tree node in the first place because it would
 prevent any driver from properly setting up the translations.
 
+Optional properties:
+--------------------
+- pasid-num-bits: Some masters support multiple address spaces for DMA, by
+  tagging DMA transactions with an address space identifier. By default,
+  this is 0, which means that the device only has one address space.
+
 
 Notes:
 ======