net: dsa: introduce a .port_probe() method in struct dsa_ops

Some drivers might want to execute code for each port at probe time, as
opposed to executing code just-in-time for the port selected for
networking.

To cater to that use case, introduce a .port_probe() callback method
into the DSA switch operations which is called for each available port,
at the end of dsa_port_probe().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Tested-by: Michael Walle <michael@walle.cc>