ethernet: arc: emac refactoring for future specific SoC glue layer devicetree bindings addition

This is a preparation of an api changes for the emac_main.c module.
The involved functions are arc_emac_probe and arc_emac_remove.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
ethernet: arc: mdio refactoring for future specific SoC glue layer devicetree bindings addition

This is a preparation of an api changes for the emac_mdio.c module.
The involved function is arc_mdio_probe.

Signed-off-by: Romain Perier <romain.perier@gmail.com>