kontron-sl-mx6ul: switch to DM_SERIAL

Enable DM_SERIAL to avoid board removal warning.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
diff --git a/configs/kontron-sl-mx6ul_defconfig b/configs/kontron-sl-mx6ul_defconfig
index 12d96b6..f295ecf 100644
--- a/configs/kontron-sl-mx6ul_defconfig
+++ b/configs/kontron-sl-mx6ul_defconfig
@@ -90,6 +90,7 @@
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_CONS_INDEX=4
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y