ipmi: New tunable to deactivate IPMI watchdog

In case we want to use other WD than IPMI-provided, add
sysctl to disable initialization.

Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D31548

(cherry picked from commit e8ad0a0059afe1cd0af39bab49018ae7bc9be937)