Documentation: networking: add a DSA document

Describe how the DSA subsystem works, its design principles,
limitations, and describe in details how to implement a DSA switch
driver.
Acked-by: NAndrew Lunn <andrew@lunn.ch>
Acked-by: NScott Feldman <sfeldma@gmail.com>
Reviewed-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>