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>