net: dsa: mv88e6xxx: Add support for ethernet switch 88E6341

The Marvell 88E6341 device is single-chip, 6-port Ethernet switch with
four integrated 10/100/1000Mbps Ethernet transceivers and one high speed
SerDes interfaces. It is partially compatible with switches of family
88E6352 and switches of family 88E6390.

This commit adds an initial support for this switch by describing its
capabilities to the driver and introducing a new family.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>