net: dsa: mv88e6xxx: Add support for Marvell 88E6240

The Marvell 88E6240 has been tested successfully without further
changes. Add entry to the table of supported devices.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>