documentation: networking: devlink: add prestera switched driver Documentation

Add documentation for the devlink feature prestera switchdev driver supports:
add description for the support of the driver-specific devlink traps
(include both traps with action TRAP and action DROP);

Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>