diff --git a/Documentation/devicetree/bindings/mailbox/apple,mailbox.yaml b/Documentation/devicetree/bindings/mailbox/apple,mailbox.yaml
index 2c1704b34e7aa3cd9177702f6cb85571557765ca..c4255f42e8019801c0cc3c76fbb985765a365670 100644
--- a/Documentation/devicetree/bindings/mailbox/apple,mailbox.yaml
+++ b/Documentation/devicetree/bindings/mailbox/apple,mailbox.yaml
@@ -56,6 +56,9 @@ properties:
   "#mbox-cells":
     const: 0
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg