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/ls1021atwr_sdcard_qspi_defconfig b/configs/ls1021atwr_sdcard_qspi_defconfig
index 2743848..e3e64f9 100644
--- a/configs/ls1021atwr_sdcard_qspi_defconfig
+++ b/configs/ls1021atwr_sdcard_qspi_defconfig
@@ -60,6 +60,7 @@
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
 CONFIG_PHY_GIGE=y
 CONFIG_E1000=y
 CONFIG_MII=y