Ethernet RMII driver

This is the driver for the Ethernet accessory board with LAN8720 PHY chip. It can send and receive raw Ethernet packets with 100Mbit/s full duplex without any bottlenecks. To achieve this it basically bit-bangs the RMII protocol (Reduced Media Independant Interface) directly to the PHY chip. The accessory board (see forum thread) also has a 24AA025E48 ROM for the MAC address and a 25MHz crystal on board.