devlink: introduce line card device info infrastructure

Extend the line card info message with information (e.g., FW version)
about devices found on the line card.

Example:
$ devlink lc info pci/0000:01:00.0 lc 8
pci/0000:01:00.0:
  lc 8
    versions:
        fixed:
          hw.revision 0
        running:
          ini.version 4
    devices:
      device 0
        versions:
            running:
              fw 19.2010.1310
      device 1
        versions:
            running:
              fw 19.2010.1310
      device 2
        versions:
            running:
              fw 19.2010.1310
      device 3
        versions:
            running:
              fw 19.2010.1310

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>