net: ethernet: Add DT bindings for the Gemini ethernet

This adds the device tree bindings for the Gemini ethernet
controller. It is pretty straight-forward, using standard
bindings and modelling the two child ports as child devices
under the parent ethernet controller device.

Cc: devicetree@vger.kernel.org
Cc: Tobias Waldvogel <tobias.waldvogel@gmail.com>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>