[CPUFREQ] ARM: ux500: send cpufreq notification for all cpus

The same clock is used for all cpus so we must notify the frequency change
for each one in order to update the configuration of all twd clockevents.

change since V1:
* use policy->cpus instead of cpu_online_mask

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Dave Jones <davej@redhat.com>