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>