Document struct thermal_zone_device and struct thermal_governor fields
and their use by the thermal framework code.

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