netdevsim: extend device attrs to support port addition and deletion

In order to test flows in core, it is beneficial to maintain previously
supported possibility to add and delete ports during netdevsim lifetime.
Do it by extending device sysfs attrs by "new_port" and "del_port".

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