dt-bindings: net: dsa: b53: Add missing size and address cells to example

Add the missing size and address cells to the b53 example. Otherwise, it may not
compile or issue warnings if directly copied into a device tree.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>