bnxt_re: Add support to display board_id in ibv_devinfo output

Added support to display board_id in ibv_devinfo output.
ibv_devinfo util reads the board_id from below sysctl
attribute, so added this sysctl attribute.

sys.class.infiniband.bnxt_reX.board_id

Reviewed by: ssaxena
Differential Revision: https://reviews.freebsd.org/D54524
MFC after: 3 days