devlink: introduce line card devices support

Line card can contain one or more devices that makes sense to make
visible to the user. For example, this can be a gearbox with
flash memory, which could be updated.

Provide the driver possibility to attach such devices to a line card
and expose those to user.

Example:
$ devlink lc show pci/0000:01:00.0 lc 8
pci/0000:01:00.0:
  lc 8 state active type 16x100G
    supported_types:
      16x100G
    devices:
      device 0
      device 1
      device 2
      device 3

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>