net: devlink: introduce nested devlink entity for line card

For the purpose of exposing device info and allow flash update which is
going to be implemented in follow-up patches, introduce a possibility
for a line card to expose relation to nested devlink entity. The nested
devlink entity represents the line card.

Example:

$ devlink lc show pci/0000:01:00.0 lc 1
pci/0000:01:00.0:
  lc 1 state active type 16x100G nested_devlink auxiliary/mlxsw_core.lc.0
    supported_types:
       16x100G
$ devlink dev show auxiliary/mlxsw_core.lc.0
auxiliary/mlxsw_core.lc.0

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>