CHROMIUM: power: Allow bus used for power supply to be found

CHROMIUM: power: Allow bus used for power supply to be found

Some power supplies such as batteries are connected via a bus such as i2c.
When we want to resume the battery we must resume the bus also, otherwise
it will not be possible to communicate with it.

Add a function which returns the device that a power supply is connected
on, to permit a caller to resume the bus also.

BUG=chrome-os-partner:10617
TEST=build and boot to kernel prompt on snow
Use charger-manager, with other changes, and see that in the
suspend_again() handler, the battery temperature can be read successfully.
This indicates that the i2c bus is working properly.
Change-Id: I1f19606861e0cb7f86621c2ad22415d7761cb060
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43012
Reviewed-by: Benson Leung <bleung@chromium.org>