configs: enable DM_MDIO for LS1021A-TWR and LS1021A-TSN

The tsec driver now requires DM_MDIO when DM_ETH is enabled. To avoid
build errors, enable DM_MDIO in these boards' configs before we actually
add DM_MDIO support to tsec.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/configs/ls1021atsn_sdcard_defconfig b/configs/ls1021atsn_sdcard_defconfig
index 8046f64..49197ee 100644
--- a/configs/ls1021atsn_sdcard_defconfig
+++ b/configs/ls1021atsn_sdcard_defconfig
@@ -55,6 +55,7 @@
 CONFIG_PHY_BROADCOM=y
 CONFIG_PHY_FIXED=y
 CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
 CONFIG_PHY_GIGE=y
 CONFIG_MII=y
 CONFIG_TSEC_ENET=y