dt-bindings: net: dsa: marvell: add "marvell,mv88e6250" compatible string

The mv88e6250 has port_base_addr 0x8 or 0x18 (depending on
configuration pins), so it constitutes a new family and hence needs
its own compatible string.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>