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>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 5c2910c..82bd919 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -538,6 +538,17 @@
 		compatible = "sandbox,smem";
 	};
 
+	sound {
+		compatible = "sandbox,sound";
+		cpu {
+			sound-dai = <&i2s 0>;
+		};
+
+		codec {
+			sound-dai = <&audio 0>;
+		};
+	};
+
 	spi@0 {
 		#address-cells = <1>;
 		#size-cells = <0>;