dt-bindings: power: supply: New bindings for LEGO MINDSTORMS EV3 battery

This add a new device tree binding for LEGO MINDSTORMS EV3 battery. The EV3
has some built-in capability for monitoring the attached battery.

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