ARM: da850-lego-ev3: Add device tree node for sound

This adds a device tree node for sound on LEGO MINDSTORMS EV3. The EV3
uses one of the SoC PWMs connected to an amplifier to create sound from
a speaker.

The PWM is passed through a low-pass filter, so it is actually possible
to do PCM playback, but there is no existing driver, so just using
pwm-beeper for now, since it is also a compatible mode of operation.

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