devlink: implement line card active state

Allow driver to mark a line card as active. Expose this state to the
userspace over devlink netlink interface with proper notifications.
'active' state means that line card was plugged in after
being provisioned.

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>