kernel: Import the IPMI driver from FreeBSD.

Many thanks to Markus Pfeiffer <profmakx@dragonflybsd.org> for testing,
bug hunting and fixing the watchdog (this will be pushed in a separate
commit).
sys/dev/misc/ipmi: Implement watchdog functionality

Unfortunately wdog(9) cannot handle how ipmi's watchdog works, so
we roll our own callout based solution.