ipmi: add oem message handling

Enable userspace to receive messages that a BMC transmits using an OEM
medium.  This is used by the HP iLO2.

Based on code originally written by Patrick Schoeller.

Signed-off-by: dann frazier <dannf@hp.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>