devlink: Add macro for "fw.mgmt.api" to info_get cb.

Add definition and documentation for the new generic info
"fw.mgmt.api". This macro specifies the version of the software
interfaces between driver and firmware.

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jacob Keller <jacob.e.keller@intel.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>