configs: P2020RDB: Enable DM_ETH config

Enable the DM_ETH and DM_MDIO config.

On P2020RDB, the eTSEC1 is connecting with a switch VSC7385,
so also enable the fixed PHY support.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>