thermal: devfreq_cooling: add new interface for direct power read

This patch introduces a new interface for device drivers connected to
devfreq_cooling in the thermal framework: get_real_power().

Some devices have more sophisticated methods (like power counters)
to approximate the actual power that they use.
In the previous implementation we had a pre-calculated power
table which was then scaled by 'utilization'
('busy_time' and 'total_time' taken from devfreq 'last_status').

With this new interface the driver can provide more precise data
regarding actual power to the thermal governor every time the power
budget is calculated. We then use this value and calculate the real
resource utilization scaling factor.

Reviewed-by: Chris Diamand <chris.diamand@arm.com>
Acked-by: Javi Merino <javi.merino@kernel.org>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>