dt-bindings: net: dsa: b53: Add missing reg property to example

The switch has a certain MDIO address and this needs to be specified using the
reg property. Add it to the example.
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/b53.txt

@@ -106,6 +106,7 @@ Ethernet switch connected via MDIO to the host, CPU port wired to eth0:
 
 		switch0: ethernet-switch@30 {
 			compatible = "brcm,bcm53125";
+			reg = <30>;
 			#address-cells = <1>;
 			#size-cells = <0>;