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: Brandon Streiff <brandon.streiff@ni.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>