net: dsa: Add plumbing for port mirroring

Add necessary plumbing at the slave network device level to have switch
drivers implement ndo_setup_tc() and most particularly the cls_matchall
classifier. We add support for two switch operations:

port_add_mirror and port_del_mirror() which configure, on a per-port
basis the mirror parameters requested from the cls_matchall classifier.

Code is largely borrowed from the Mellanox Spectrum switch driver.

Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>