docs: net: dsa: update probing documentation

Since the blamed commit we don't have register_switch_driver() and
unregister_switch_driver() anymore. Additionally, the expected
dsa_register_switch() and dsa_unregister_switch() calls aren't
documented.

Update the probing section with the details of how things are currently
done.

Fixes: 93e86b3b ("net: dsa: Remove legacy probing support")
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>