net: dsa: b53: implement port isolation support

net: dsa: b53: implement port isolation support

Implement port isolation support via the Protected Ports register.

Protected ports can only communicate with unprotected ports, but not
with each other, matching the expected behaviour of isolated ports.

Tested on BCM963268BU.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20251013152834.100169-1-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>