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/smdk5250_defconfig b/configs/smdk5250_defconfig
index 2151e89..58e0516 100644
--- a/configs/smdk5250_defconfig
+++ b/configs/smdk5250_defconfig
@@ -5,3 +5,4 @@
 CONFIG_DEFAULT_DEVICE_TREE="exynos5250-smdk5250"
 CONFIG_SOUND=y
 CONFIG_CMD_SOUND=y
+CONFIG_I2S=y