configs: ls1088a: enable CMD_MDIO

Enable the CMD_MDIO Kconfig option by removing the "is not set"
indication from all the defconfigs for this SoC.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>