ethernet: arc: Add support for Rockchip SoC glue 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 changes.

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