thermal: cpufreq_cooling: Use new thermal pressure update function

Thermal pressure provides a new API, which allows to use CPU frequency
as an argument. That removes the need of local conversion to capacity.
Use this new function and remove old conversion code.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>