thermal: cpu_cooling: Make 'notify_device' static

Silences the following sparse warning:

drivers/thermal/cpu_cooling.c:67:31: warning:
symbol 'notify_device' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>