pico-pi-imx6ul: Connvert to DM_SERIAL

The conversion to DM_SERIAL is mandatory, so select this option.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/pico-pi-imx6ul_defconfig b/configs/pico-pi-imx6ul_defconfig
index 1eee1e6..3af968d 100644
--- a/configs/pico-pi-imx6ul_defconfig
+++ b/configs/pico-pi-imx6ul_defconfig
@@ -70,6 +70,7 @@
 CONFIG_DM_REGULATOR_PFUZE100=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_SPL_USB_HOST=y