ppc6xx_defconfig reveals this:

drivers/built-in.o: In function `hidinput_cleanup_battery': drivers/hid/hid-input.c:351: undefined reference to`power_supply_unregister'
drivers/built-in.o: In function `hidinput_setup_battery': drivers/hid/hid-input.c:338: undefined reference to `power_supply_register'
make[1]: *** [.tmp_vmlinux1] Error 1

The defconfig in question doens't mention either option and kbuild is
genertaing

	CONFIG_HID_BATTERY_STRENGTH=y
	CONFIG_POWER_SUPPLY=m

which is wrong. Put a proper dependency in place.
Reported-by: NTony Breeds <tony@bakeyournoodle.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>