devlink: Add support for board.serial_number to info_get cb.

Board serial number is a serial number, often available in PCI
*Vital Product Data*.

Also, update devlink-info.rst documentation file.

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