Thermal: Rename thermal_instance.node to thermal_instance.tz_node.

thermal_instance should be referenced by both thermal zone devices
and thermal cooling devices.

Rename thermal_instance.node to thermal_instance.tz_node in this patch
and thermal_instanace.cdev_node will be introduced in next patch.

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>