[POWERPC] Add PMI driver for cell blade

This adds driver code for the PMI device found in future IBM products.
PMI stands for "Platform Management Interrupt" and is a way to
communicate with the BMC (Baseboard Management Controller).
It provides bidirectional communication with a low latency.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Acked-by: Heiko J Schick <schickhj@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>