arm: dts: ls1028a: declare the fixed-link speeds for the internal ENETC ports

To comply with the device tree bindings expectations for an Ethernet
controller, as well as to simplify the driver code, declare fixed-link
nodes for the internal ENETC ports (attached to the mscc_felix switch).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>