dm: sandbox: Update sound to use two buffers

dm: sandbox: Update sound to use two buffers

At present we use a single buffer for sound which means we cannot be
playing one sound while queueing up the next. This wouldn't matter except
that a long sound (more than a second) has to be created as a single
buffer, thus using a lot of memory. To better mimic what real sound
drivers do, add support for double buffering in sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>