configs: ls1028a: enable OF_LIBFDT_OVERLAY

LS1028A QDS board supports multiple combinations of SerDes protocols and
multiple add-on cards with Ethernet PHYs.  Some of them require specific
configuration in Linux DT, and the plan is to use DT overlays for them.

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