ethernet: arc: Add support for Rockchip SoC layer device tree bindings

This patch defines a platform glue layer for Rockchip SoCs which
support arc-emac driver. It ensures that regulator for the rmii is on
before trying to connect to the ethernet controller. It applies right
speed and mode changes to the grf when ethernet settings change.

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