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_defconfig b/configs/pico-imx7d_defconfig
index 13cc97b..1a818ef 100644
--- a/configs/pico-imx7d_defconfig
+++ b/configs/pico-imx7d_defconfig
@@ -83,6 +83,7 @@
 CONFIG_POWER_LEGACY=y
 CONFIG_POWER_PFUZE3000=y
 CONFIG_POWER_I2C=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y