ICSSG can operating in switch mode with 2 ext port and 1 host port with
VLAN/FDB/MDB and STP offloading. Add switchdev based driver to
support the same.

Driver itself will be integrated with icssg_prueth in future commits
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>