docs: net: dsa: document port_setup and port_teardown

These methods were added without being documented, fix that.

Fixes: fd292c18 ("net: dsa: tear down devlink port regions when tearing down the devlink port on error")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>