[POWERPC] cell: use pmi in cpufreq driver

The new PMI driver was added in order to support
cpufreq on blades that require the frequency to
be controlled by the service processor, so use it
on those.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>