dt-bindings: net: mdio: Add a nodename pattern

The node name of an MDIO controller should be MDIO. Let's add a rule for
this.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/mdio.yaml

@@ -18,6 +18,9 @@ description:
   a device specific binding document.
 
 properties:
+  $nodename:
+    pattern: "^mdio(@.*)?"
+
   reset-gpios:
     maxItems: 1
     description: