ethernet: arc: mdio changes for future SoC glue layer devtree support

This is an api changes for the emac_mdio.c module.
It will be required later when arc_emac_probe/arc_emac_remove
will no longer use 'struct platform_device'.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>