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/board/ti/am335x/Kconfig b/board/ti/am335x/Kconfig
index 1ddbb2c..d6581ac 100644
--- a/board/ti/am335x/Kconfig
+++ b/board/ti/am335x/Kconfig
@@ -37,4 +37,14 @@
 	  booted via NOR.  In this case we will enable certain pinmux early
 	  as the ROM only partially sets up pinmux.  We also default to using
 	  NOR for environment.
+
+config DM
+	default y if !SPL_BUILD
+
+config DM_GPIO
+	default y if DM && !SPL_BUILD
+
+config DM_SERIAL
+	default y if DM && !SPL_BUILD
+
 endif