pico-imx7d: Convert to CONFIG_DM_SERIAL

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

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/pico-imx7d_bl33_defconfig b/configs/pico-imx7d_bl33_defconfig
index 35e6f1d..2dcbb43 100644
--- a/configs/pico-imx7d_bl33_defconfig
+++ b/configs/pico-imx7d_bl33_defconfig
@@ -80,6 +80,7 @@
 CONFIG_POWER_PFUZE3000=y
 CONFIG_POWER_I2C=y
 CONFIG_CONS_INDEX=4
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y