dm: mx6: Move driver model CONFIGs to Kconfig

Remove driver model CONFIGs from the board config headers and use Kconfig
instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 3c0d64f..d2ebf69 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -2,3 +2,6 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL,SPL"
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_CM_FX6=y
+CONFIG_DM=y
+CONFIG_DM_GPIO=y
+CONFIG_DM_SERIAL=y