dt-bindings: host1x: Add iommu-map property

Add schema information for specifying context stream IDs. This uses
the standard iommu-map property.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>