power: supply: Allow charger manager can be built as a module

Allow charger manager can be built as a module like other charger
drivers.

Signed-off-by: Baolin Wang <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/supply/Kconfig

···
 	  called gpio-charger.
 
 config CHARGER_MANAGER
-	bool "Battery charger manager for multiple chargers"
+	tristate "Battery charger manager for multiple chargers"
 	depends on REGULATOR
 	select EXTCON
 	help

include/linux/power/charger-manager.h

···
 	u64 charging_end_time;
 };
 
-#ifdef CONFIG_CHARGER_MANAGER
+#if IS_ENABLED(CONFIG_CHARGER_MANAGER)
 extern void cm_notify_event(struct power_supply *psy,
 				enum cm_event_types type, char *msg);
 #else