MAINTAINERS: add entry for LEGO MINDSTORMS EV3

This adds an entry to MAINTAINERS for LEGO MINDSTORMS EV3 (an ARM-based
robotics platform). The files listed are exclusive to this device. Add
me as reviewer so that I will be cc'ed for any changes to these files.

Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>