hw/arm/allwinner-a10: initialize EMAC

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
