power_supply: allow a power supply to explicitly point to powered device

If a power supply has a scope of "Device", then allow the power supply
to indicate what device it actually powers. This is represented in the
power supply's sysfs directory as a symlink named "powers", which points to
the sysfs directory of the powered device.

If the device has children, then the sub-devices are also powered by
the same power supply.

Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Richard Hughes <richard@hughsie.com>