sandbox: exynos: Move CONFIG_SOUND_WM8994 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 8dd6b95..0e7b868 100644
--- a/configs/smdk5250_defconfig
+++ b/configs/smdk5250_defconfig
@@ -8,3 +8,4 @@
 CONFIG_I2S=y
 CONFIG_I2S_SAMSUNG=y
 CONFIG_SOUND_MAX98095=y
+CONFIG_SOUND_WM8994=y