pico-imx6: 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/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index ed15d32..300bb61 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -82,6 +82,7 @@
 CONFIG_MII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_USB_GADGET=y