[POWERPC] cbe_cpufreq: Limit frequency via cpufreq notifier chain

... and get rid of cpufreq_set_policy call that caused a build
failure due interfering commits.

Signed-off-by: Thomas Renninger <trenn@suse.de>
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>