devlink: add support to create line card and expose to user

Extend the devlink API so the driver is going to be able to create and
destroy linecard instances. There can be multiple line cards per devlink
device. Expose this new type of object over devlink netlink API to the
userspace, with notifications.

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