selftests: devlink_lib: Add devlink port helpers

Add two devlink port helpers:
 * devlink port get by netdev
 * devlink cpu port get

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