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 <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

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

···
     items:
       - const: core
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg