thermal: devfreq_cooling: use a thermal_cooling_device for register and unregister

Be consistent with what other cooling devices do and return a struct
thermal_cooling_device * on register.  Also, for the unregister, accept
a struct thermal_cooling_device * as parameter.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>