imx: imx8mm_mx8menlo: Enable DM_SERIAL

Enable CONFIG_DM_SERIAL. uart2 and its pinmux was already
marked with u-boot,dm-spl.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/imx8mm-mx8menlo_defconfig b/configs/imx8mm-mx8menlo_defconfig
index 0d3e19e..a416495 100644
--- a/configs/imx8mm-mx8menlo_defconfig
+++ b/configs/imx8mm-mx8menlo_defconfig
@@ -105,6 +105,7 @@
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_SYSRESET=y
 CONFIG_SPL_SYSRESET=y