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>

Documentation/devicetree/bindings/net/dsa/marvell.txt

@@ -21,10 +21,13 @@ which is at a different MDIO base address in different switch families.
 			  6341, 6350, 6351, 6352
 - "marvell,mv88e6190"	: Switch has base address 0x00. Use with models:
 			  6190, 6190X, 6191, 6290, 6390, 6390X
+- "marvell,mv88e6250"	: Switch has base address 0x08 or 0x18. Use with model:
+			  6250
 
 Required properties:
-- compatible		: Should be one of "marvell,mv88e6085" or
-			  "marvell,mv88e6190" as indicated above
+- compatible		: Should be one of "marvell,mv88e6085",
+			  "marvell,mv88e6190" or "marvell,mv88e6250" as
+			  indicated above
 - reg			: Address on the MII bus for the switch.
 
 Optional properties: