dt-bindings: net: dsa: marvell: fix compatible in example

While the MV88E6390 switch chip exists, one is supposed to use a
compatible of "marvell,mv88e6190" for it. Fix this in the given example.

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Fixes: a3c53be5 ("net: dsa: mv88e6xxx: Support multiple MDIO busses")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>