mlxsw: core_linecards: Add line card objects and implement provisioning

Introduce objects for line cards and an infrastructure around that.
Use devlink_linecard_create/destroy() to register the line card with
devlink core. Implement provisioning ops with a list of supported
line cards.

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>