power: supply: New driver for LEGO MINDSTORMS EV3 battery

This adds a new driver for the LEGO MINDSTORMS EV3 battery. The EV3 is
an embedded ARM device that can use 6 AA batteries or a special rechargeable
Li-ion battery pack. The rechargeable battery pack presses a special key
switch in the battery compartment to indicate that it is present.

The EV3 is only capable of monitoring battery voltage and current. The
charging circuit is built into the rechargeable battery pack and there is
no way to communicate with is, so we can't provide any information about
charging status.

When not using the rechargeable battery pack, it is most common to use
alkaline batteries to power the device, but it is also common for people to
use rechargeable NiMH batteries. Since there is not a way to automatically
differentiate between these, the technology property is made writable.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>