thermal: cpu_cooling: find max level during device registration

CPU frequency tables don't update after the driver is registered and so we don't
need to iterate over them to find total number of states every time
cpufreq_get_max_state() is called. Do it once at boot time.

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