net: dsa: mv88e6xxx: add MV88E6097 switch

Add support for the MV88E6097 switch. The change was tested on an Armada
based platform with a MV88E6097 switch.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@netmodule.com>
Signed-off-by: David S. Miller <davem@davemloft.net>