dm: omap3: 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/mcx_defconfig b/configs/mcx_defconfig
index c2031f8..6566d40 100644
--- a/configs/mcx_defconfig
+++ b/configs/mcx_defconfig
@@ -2,3 +2,6 @@
 +S:CONFIG_ARM=y
 +S:CONFIG_OMAP34XX=y
 +S:CONFIG_TARGET_MCX=y
+CONFIG_DM=n
+CONFIG_DM_SERIAL=n
+CONFIG_DM_GPIO=n