drm/panfrost: Register devfreq cooling and attempt to add Energy Model

Register devfreq cooling device and attempt to register Energy Model. This
will add the devfreq device to the Energy Model framework. It will create
a dedicated and unified data structures used i.e. in thermal framework.
The last NULL parameter indicates that the power model is simplified and
created based on DT 'dynamic-power-coefficient', voltage and frequency.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>