arm64: dts: ls1028a-qds: define mdio slots for networking options

The ls1028a QDS board support different pluggable PHY cards.  Define the
nodes for these slots to be updated at boot time with overlay according
to board setup.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>