dm: at91: snapper: 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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 2faae15..66c44cc 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -1,3 +1,6 @@
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20"
 CONFIG_ARM=y
 CONFIG_TARGET_SNAPPER9260=y
+CONFIG_DM=y
+CONFIG_DM_GPIO=y
+CONFIG_DM_SERIAL=y