Thermal: rename structure thermal_cooling_device_instance to thermal_instance

This struct is used to describe the behavior for a thermal
cooling device on a certain trip point for a certain thremal zone.

thermal_cooling_device_instance is not accurate, as a cooling device
can be used for more than one trip point in one thermal zone device.

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>