thermal: cpu_cooling: don't iterate over all allowed_cpus to update cpufreq policy

All CPUs present in 'allowed_cpus' share the same 'struct cpufreq_policy'
structure and so calling cpufreq_update_policy() for each of them doesn't make
sense.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>