ipmi: add powerpc openfirmware sensing

Add support for of_platform_driver to the ipmi_si module.  When loading the
module, the driver will be registered to of_platform.  The driver will be
probed for all devices with the type ipmi.  It's supporting devices with
compatible settings ipmi-kcs, ipmi-smic and ipmi-bt.  Only ipmi-kcs could be
tested.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Acked-by: Heiko J Schick <schihei@de.ibm.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>