ipmi: add options to disable openfirmware and PCI scanning

Add try...  parameters to disable pci and platform (openfirmware) device
scanning for IPMI.  Also add docs for all the try...  parameters.

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>