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: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>