thermal: allow u8500-thermal driver to be a module

When the thermal subsystem is a loadable module, the u8500 driver
fails to build:

drivers/thermal/built-in.o: In function `db8500_thermal_probe':
db8500_thermal.c:(.text+0x96c): undefined reference to `thermal_zone_device_register'
drivers/thermal/built-in.o: In function `db8500_thermal_work':
db8500_thermal.c:(.text+0xab4): undefined reference to `thermal_zone_device_update'

This changes the symbol to a tristate, so Kconfig can track the
dependency correctly.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/mfd/db8500-prcmu.c

@@ -2048,6 +2048,7 @@ int db8500_prcmu_config_hotmon(u8 low, u8 high)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(db8500_prcmu_config_hotmon);
 
 static int config_hot_period(u16 val)
 {
@@ -2074,11 +2075,13 @@ int db8500_prcmu_start_temp_sense(u16 cycles32k)
 
 	return config_hot_period(cycles32k);
 }
+EXPORT_SYMBOL_GPL(db8500_prcmu_start_temp_sense);
 
 int db8500_prcmu_stop_temp_sense(void)
 {
 	return config_hot_period(0xFFFF);
 }
+EXPORT_SYMBOL_GPL(db8500_prcmu_stop_temp_sense);
 
 static int prcmu_a9wdog(u8 cmd, u8 d0, u8 d1, u8 d2, u8 d3)
 {

drivers/thermal/Kconfig

@@ -237,8 +237,8 @@ config DOVE_THERMAL
 	  framework.
 
 config DB8500_THERMAL
-	bool "DB8500 thermal management"
-	depends on ARCH_U8500
+	tristate "DB8500 thermal management"
+	depends on MFD_DB8500_PRCMU
 	default y
 	help
 	  Adds DB8500 thermal management implementation according to the thermal