mx53loco: Add DM_SERIAL support

The conversion to DM_SERIAL is mandatory, so add support
for it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/mx53loco_defconfig b/configs/mx53loco_defconfig
index d5f2b70..e332c93 100644
--- a/configs/mx53loco_defconfig
+++ b/configs/mx53loco_defconfig
@@ -61,6 +61,7 @@
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_POWER_FSL=y
 CONFIG_POWER_I2C=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_MX5=y