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/test/dm/Makefile b/test/dm/Makefile
index ef450b7..28ede9a 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -22,6 +22,7 @@
 obj-$(CONFIG_DM_GPIO) += gpio.o
 obj-$(CONFIG_DM_HWSPINLOCK) += hwspinlock.o
 obj-$(CONFIG_DM_I2C) += i2c.o
+obj-$(CONFIG_DM_SOUND) += i2s.o
 obj-$(CONFIG_LED) += led.o
 obj-$(CONFIG_DM_MAILBOX) += mailbox.o
 obj-$(CONFIG_DM_MMC) += mmc.o