ynl: devlink: add missing board-serial-number

Add a missing attribute of board serial number.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://patch.msgid.link/20250320085947.103419-2-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>