ARM: da850-lego-ev3: Add device tree node for A/DC

This adds a node for the TI ADS7957 analog/digital converter on LEGO
MINDSTORMS EV3 as well as a regulator node that is used by the A/DC node.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>