net: dsa: mv88e6131: Add support for mv88e6185

The mv88e6185 is part of the family that the mv88e6131 driver
supports. Add it to the probe function, and set the number of ports.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>