devlink: Add layer 3 control packet traps

Add layer 3 control packet traps such as ARP and DHCP, so that capable
device drivers could register them with devlink. Add documentation for
every added packet trap and packet trap group.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>