dts: bindings: ethernet: Remove fixed link

Remove ethernet-fixed-link binding as it is redundant with the
phy bindings. Clearly, ethernet does not work without the L1
layer, which is a phy device, or an integrated mac/phy device,
and all of these things should be described properly in DT.
The schema did not even come with a compatible, meaning nodelabels
were hardcoded into the drivers, which is unacceptable.

- Remove the binding file for ethernet-fixed-link.yaml.
- Remove fixed link functionality from the nxp s32 gmac driver.
  Since this functionality is already covered by the phy support,
  it is redundant.
- Remove fixed link include from the s32 gmac binding.
- Remove fixed link include from the nuvoton numaker binding.
  As far as I can tell the corresonding driver does not even
  use it anyways, and I did not find any board with this device
  that describes a "fixed link".
- Move the definition into the nxp,kinetis-ethernet binding
  as the eth_mcux driver, which is already being deprecated,
  does use this, contain the debt to the legacy driver.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>