ipmi: split device discovery and registration

The ipmi spec indicates that we should only make use of one si per bmc, so
separate device discovery and registration to make that possible.

[thenzl@redhat.com: fix mutex use]
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>