Thermal: Rename thermal_zone_device.cooling_devices

Rename thermal_zone_device.cooling_devices
to thermal_zone_device.thermal_instances

thermal_zone_device.cooling_devices is not accurate
as this is a list for thermal instances, rather than cooling devices.

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>