HID: logitech-hidpp: add a sysfs file to tell we support power_supply

This way, upower can add a simple udev rule to decide whether or not
it should use the internal unifying support or just the generic kernel
one.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>