bnxt_en: Add partno to devlink info_get cb

Add part number info from the vital product data to info_get command
via devlink tool. Update bnxt.rst documentation as well.

Cc: Jakub Kicinski <kuba@kernel.org>
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>