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>
ethernet: arc: Add support for specific SoC layer device tree bindings

Some platforms have special bank registers which might be used to
select the correct clock or the right mode for Media Indepent Interface
controllers. Sometimes, it is also required to activate vcc regulators
in the right order to supply the ethernet controller at the right time.
This patch is an architecture refactoring of the arc-emac device driver.
It adds a new software design which allows to add specific platform
glue layer. Each platform has now its own module which performs custom
initialization and remove for the target and then calls to the
core driver.

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