[POWERPC] Fix PMI breakage in cbe_cbufreq driver

The recent change to cell_defconfig to enable cpufreq on Cell exposed
the fact that the cbe_cpufreq driver currently needs the PMI interface
code to compile, but Kconfig doesn't make sure that the PMI interface
code gets built if cbe_cpufreq is enabled.

In fact cbe_cpufreq can work without PMI, so this ifdefs out the code
that deals with PMI.  This is a minimal solution for 2.6.22; a more
comprehensive solution will be merged for 2.6.23.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>