Thermal: List thermal_instance in thermal_cooling_device.

List thermal_instance in thermal_cooling_device so that
cooling device can know the cooling state requirement
of all the thermal instances.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Eduardo Valentin <eduardo.valentin@ti.com>