ARM: imx: imx8mn-evk: enable DM_SERIAL

U-Boot complains that CONFIG_SERIAL is not converted to CONFIG_DM_SERIAL
and gives a deadline before possibly removing the board. Migrate to
DM_SERIAL to fulfill the request.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/imx8mn_evk_defconfig b/configs/imx8mn_evk_defconfig
index 7749ebe..b2981d1 100644
--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -83,6 +83,8 @@
 CONFIG_SPL_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_SERIAL=y
+# CONFIG_SPL_DM_SERIAL is not set
 CONFIG_MXC_UART=y
 CONFIG_SYSRESET=y
 CONFIG_SPL_SYSRESET=y