mlxsw: core_linecards: Introduce ops for linecards status change tracking

Introduce an infrastructure allowing users to register a set
of operations which are to be called whenever a line card gets
active/inactive.

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