dt-bindings: net: dsa: b53: Specify unit address in hex

The unit address should be 1e, because the unit address is supposed
to be in hexadecimal.

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