dt-bindings: arm-smmu: Document nvidia,memory-controller property

On NVIDIA SoC's the ARM SMMU needs to interact with the memory
controller in order to map memory clients to the corresponding stream
IDs. Document how the nvidia,memory-controller property can be used to
achieve this.

Note that this is a backwards-incompatible change that is, however,
necessary to ensure correctness. Without the new property, most of the
devices would still work but it is not guaranteed that all will.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20220429082243.496000-2-thierry.reding@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>