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>