dm: sound: Create a uclass for sound

dm: sound: Create a uclass for sound

The sound driver pulls together the audio codec and i2s drivers in order
to actually make sounds. It supports setup() and play() methods. The
sound_find_codec_i2s() function allows locating the linked codec and i2s
devices. They can be referred to from uclass-private data.

Add a uclass and a test for sound.

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