dt-bindings: net: dsa: sja1105: fix example so all ports have a phy-handle of fixed-link

All ports require either a phy-handle or a fixed-link, and port 3 in the
example didn't have one. Add it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>