Thermal: Make thermal_cdev_update as a global function

This patch makes the thermal_cdev_update function as a
global one, so that other files can use it. This function
serves as a single arbitrator to set the state of a cooling
device.

Signed-off-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>