[CELL] cbe_cpufreq: reorganize code

This patch reorganizes the code of the driver into three files.
Two cbe_cpufreq_pmi.c and cbe_cpufreq_pervasive.c care about hardware.
cbe_cpufreq.c contains the logic.
There is no changed behaviour, except that the PMI related function
is now located in a seperate module cbe_cpufreq_pmi. This module
will be required by cbe_cpufreq, if CONFIG_CBE_CPUFREQ_PMI has been set.

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