thermal: cpu cooling: allow module builds

As thermal drivers can be built as modules and also
the thermal framework itself, building cpu cooling
only as built-in can cause linking errors. For instance:
* Generic Thermal sysfs driver
*
Generic Thermal sysfs driver (THERMAL) [M/n/y/?] m
  generic cpu cooling support (CPU_THERMAL) [N/y/?] (NEW) y

with the following drive:
CONFIG_OMAP_BANDGAP=m

generates:
ERROR: "cpufreq_cooling_unregister" [drivers/staging/omap-thermal/omap-thermal.ko] undefined!
ERROR: "cpufreq_cooling_register" [drivers/staging/omap-thermal/omap-thermal.ko] undefined!

This patch changes cpu cooling driver to allow it
to be built as module.

Reported-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>