sandbox: exynos: Move CONFIG_I2S to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/snow_defconfig b/configs/snow_defconfig
index 8c1d64d..eb8c64a 100644
--- a/configs/snow_defconfig
+++ b/configs/snow_defconfig
@@ -9,3 +9,4 @@
 CONFIG_CMD_CROS_EC=y
 CONFIG_SOUND=y
 CONFIG_CMD_SOUND=y
+CONFIG_I2S=y