ethernet: arc: Add support for specific SoC glue 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 a refactoring of the arc-emac
device driver, it adds a new software architecture 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.

This commit is a general cleanup and preparation for the above changes.

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