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/dig297_defconfig b/configs/dig297_defconfig
index 95bc353..0d18290 100644
--- a/configs/dig297_defconfig
+++ b/configs/dig297_defconfig
@@ -1,3 +1,6 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_DIG297=y
+CONFIG_DM=n
+CONFIG_DM_SERIAL=n
+CONFIG_DM_GPIO=n