bnxt_en: Add fw.mgmt.api version to devlink info_get cb.

Display the minimum version of firmware interface spec supported
between driver and firmware. Also update bnxt.rst documentation file.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>