dm: sound: Create a uclass for i2s

The i2s bus is commonly used with audio codecs. It provides a way to
stream digital data sychronously in both directions. U-Boot only supports
audio output, so this uclass is very simple, with a single tx_data()
method.

Add a uclass and a test for i2s.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 9ced5be..5c2910c 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -47,7 +47,7 @@
 		#sound-dai-cells = <1>;
 	};
 
-		cros_ec: cros-ec {
+	cros_ec: cros-ec {
 		reg = <0 0>;
 		compatible = "google,cros-ec-sandbox";
 
@@ -378,6 +378,11 @@
 		u-boot,dm-pre-reloc;
 	};
 
+	i2s: i2s {
+		compatible = "sandbox,i2s";
+		#sound-dai-cells = <1>;
+	};
+
 	misc-test {
 		compatible = "sandbox,misc_sandbox";
 	};