dt-bindings: iommu: apple,dart: Add power-domains property

This will bind to the PMGR pwrstate nodes that control power/clock
gating to SoC blocks. The DART driver doesn't do runtime-pm yet, so
initially this will just keep the domain on permanently.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>

Documentation/devicetree/bindings/iommu/apple,dart.yaml

@@ -41,6 +41,9 @@ properties:
       Has to be one. The single cell describes the stream id emitted by
       a master to the IOMMU.
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg