power: supply: core: provide function stubs if CONFIG_POWER_SUPPLY=n

Fix build error when CONFIG_POWER_SUPPLY is not enabled.

The build error occurs in mips (cavium_octeon_defconfig).

mips-linux-gnu-ld: drivers/usb/dwc3/core.o: in function `dwc3_remove':
drivers/usb/dwc3/core.c:1657: undefined reference to `power_supply_put'
mips-linux-gnu-ld: drivers/usb/dwc3/core.o: in function `dwc3_get_properties':
drivers/usb/dwc3/core.c:1270: undefined reference to `power_supply_get_by_name'
mips-linux-gnu-ld: drivers/usb/dwc3/core.o: in function `dwc3_probe':
drivers/usb/dwc3/core.c:1632: undefined reference to `power_supply_put'

Fixes: 59fa3def35de ("usb: dwc3: add a power supply for current control")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Ray Chi <raychi@google.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

include/linux/power_supply.h

@@ -381,8 +381,14 @@ struct power_supply_battery_info {
 extern struct atomic_notifier_head power_supply_notifier;
 extern int power_supply_reg_notifier(struct notifier_block *nb);
 extern void power_supply_unreg_notifier(struct notifier_block *nb);
+#if IS_ENABLED(CONFIG_POWER_SUPPLY)
 extern struct power_supply *power_supply_get_by_name(const char *name);
 extern void power_supply_put(struct power_supply *psy);
+#else
+static inline void power_supply_put(struct power_supply *psy) {}
+static inline struct power_supply *power_supply_get_by_name(const char *name)
+{ return NULL; }
+#endif
 #ifdef CONFIG_OF
 extern struct power_supply *power_supply_get_by_phandle(struct device_node *np,
 							const char *property);