Documentation: net: dsa: sja1105: Add info about supported traffic modes

This adds a table which illustrates what combinations of management /
regular traffic work depending on the state the switch ports are in.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>