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>
diff --git a/drivers/thermal/Kconfig b/drivers/thermal/Kconfig
index 0852fd0..7c92c09 100644
--- a/drivers/thermal/Kconfig
+++ b/drivers/thermal/Kconfig
@@ -237,8 +237,8 @@
 	  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