hw/arm/mps2: Add ethernet

The MPS2 FPGA images support ethernet via a LAN9220. We use
QEMU's LAN9118 model, which is software compatible except
that it is missing the checksum-offload feature.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1500029487-14822-9-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
