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>