ARM: dts: da850-lego-ev3: Add node for LEGO MINDSTORMS EV3 Battery

This adds a new node to the LEGO MINDSTORMS EV3 device tree for the battery.

Signed-off-by: David Lechner <david@lechnology.com>
[nsekhar@ti.com: minor headline fix]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>