hw/net: add support for Allwinner EMAC Fast Ethernet controller

hw/net: add support for Allwinner EMAC Fast Ethernet controller

This patch adds support for the Fast Ethernet MAC found on Allwinner
SoCs, together with a basic emulation of Realtek RTL8201CP PHY.

Since there is no public documentation of the Allwinner controller, the
implementation is based on Linux kernel driver.

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>