net: dsa: Add a private structure pointer to dsa_port

This is supposed to share information between the driver and the tagger,
or used by the tagger to keep some state. Its use is optional.

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