configs: ls1028aqds: enable mdio muxing by default

LS1028A QDS boards have a MDIO MUX and they require the driver for it for
PHYs to work.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>