Add definition and documentation for the new generic
info "board.part_number".

The new one is for part number specific use, and board.id
is modified to match the documentation in devlink-info.
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>