dt-bindings: net: Add the old DT bindings for "fixed-link"

Per the upstream Linux kernel doc:

  Documentation/devicetree/bindings/net/ethernet-controller.yaml

There are two ways to describe a fixed PHY attached to an Ethernet
device. This updates our dt-bindings doc to add the old DT bindings.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>