net: cadence_gem: Split state struct and type into header

net: cadence_gem: Split state struct and type into header

Create a new header for Cadence GEM to allow using the device with
modern SoC programming conventions. The state struct needs to be
visible to embed the device in SoC containers.

Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: a98b5df6440c5bff8f813a26bb53ce1cfefb4c4c.1431381507.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>