Documentation: networking: dsa: Add Broadcom SF2 document

Add a document describing the Broadcom Starfigther 2 switch hardware,
its specifics, and how the driver is implemented and its specifics.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>