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>

drivers/thermal/Kconfig

@@ -67,7 +67,7 @@ config USER_SPACE
 	  Enable this to let the user space manage the platform thermals.
 
 config CPU_THERMAL
-	bool "generic cpu cooling support"
+	tristate "generic cpu cooling support"
 	depends on CPU_FREQ
 	select CPU_FREQ_TABLE
 	help

include/linux/cpu_cooling.h

@@ -29,7 +29,7 @@
 #define CPUFREQ_COOLING_START		0
 #define CPUFREQ_COOLING_STOP		1
 
-#ifdef CONFIG_CPU_THERMAL
+#if defined(CONFIG_CPU_THERMAL) || defined(CONFIG_CPU_THERMAL_MODULE)
 /**
  * cpufreq_cooling_register - function to create cpufreq cooling device.
  * @clip_cpus: cpumask of cpus where the frequency constraints will happen