thermal: cpu_cooling: create structure for idle time stats

We keep two arrays for idle time stats and allocate memory for them
separately. It would be much easier to follow if we create an array of
idle stats structure instead and allocate it once.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>