thermal: cpu_cooling: don't store cpu_dev in cpufreq_cdev

'cpu_dev' is used by only one function, get_static_power(), and it
wouldn't be time consuming to get the cpu device structure within it.
This would help removing cpu_dev from struct cpufreq_cooling_device.

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>