dt-bindings: net: dsa: marvell: describe compatibility string

There are two compatibility strings for mv88e6xxx, but it isn't clear
from the documentation why only those two exist when the mv88e6xxx driver
supports more than the 6085 and 6190. Briefly describe how the compatible
property is used, and provide guidance on which to use.

The model list comes from looking at port_base_addr values (0x0 vs 0x10)
in drivers/net/dsa/mv88e6xxx/chip.c.
Signed-off-by: NBrandon Streiff <brandon.streiff@ni.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -13,9 +13,18 @@ placed as a child node of an mdio device.
 The properties described here are those specific to Marvell devices.
 Additional required and optional properties can be found in dsa.txt.
 
+The compatibility string is used only to find an identification register,
+which is at a different MDIO base address in different switch families.
+- "marvell,mv88e6085"	: Switch has base address 0x10. Use with models:
+			  6085, 6095, 6097, 6123, 6131, 6141, 6161, 6165,
+			  6171, 6172, 6175, 6176, 6185, 6240, 6320, 6321,
+			  6341, 6350, 6351, 6352
+- "marvell,mv88e6190"	: Switch has base address 0x00. Use with models:
+			  6190, 6190X, 6191, 6290, 6390, 6390X
+
 Required properties:
 - compatible		: Should be one of "marvell,mv88e6085" or
-			  "marvell,mv88e6190"
+			  "marvell,mv88e6190" as indicated above
 - reg			: Address on the MII bus for the switch.
 
 Optional properties: