thermal: cpu_cooling: Fix power calculation when CPUs are offline

Ensure that the CPU for which the frequency is being requested
is online. If none of the CPUs are online the requested power is
returned as 0.

Acked-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Kapileshwar Singh <kapileshwar.singh@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit dd658e02357ec879edd0c62ee1f3a19c92b0b027)
Signed-off-by: Kevin Hilman <khilman@linaro.org>