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: Andrew Lunn <andrew@lunn.ch>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>