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/omap3_beagle_defconfig b/configs/omap3_beagle_defconfig
index a3e4c2c..5a2d20b 100644
--- a/configs/omap3_beagle_defconfig
+++ b/configs/omap3_beagle_defconfig
@@ -3,3 +3,6 @@
 +S:CONFIG_ARM=y
 +S:CONFIG_OMAP34XX=y
 +S:CONFIG_TARGET_OMAP3_BEAGLE=y
+CONFIG_DM=y
+CONFIG_DM_GPIO=y
+CONFIG_DM_SERIAL=y