colibri_pxa270: transition to driver model for serial

Add serial platform data to board file.
Enable driver model for PXA serial driver.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/configs/colibri_pxa270_defconfig b/configs/colibri_pxa270_defconfig
index 0b5c20d..e0a36f1 100644
--- a/configs/colibri_pxa270_defconfig
+++ b/configs/colibri_pxa270_defconfig
@@ -16,6 +16,8 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_DM=y
+CONFIG_DM_SERIAL=y
 CONFIG_PXA_SERIAL=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y