[POWERPC] cell: add cpufreq driver for Cell BE processor

This patch adds a cpufreq backend driver to enable frequency scaling on cell.

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