Add a new attribute that indicates the split ability of devlink port.

Drivers are expected to set it via devlink_port_attrs_set(), before
registering the port.
Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>