dt-bindings: rng: meson: add optional power-domains

On newer SoCs, the random number generator can require a power-domain to
operate, add it as optional.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/amlogic,meson-rng.yaml

@@ -26,6 +26,9 @@ properties:
     items:
       - const: core
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg