net: dsa: lan9303: Added Documentation/networking/dsa/lan9303.txt

Provide a rough overview of the state of the driver. And explain that the
driver operates in two modes: bridged and port-separated.

Signed-off-by: Egil Hjelmeland <egil.hjelmeland@zenitel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>