net: ethernet: wiznet: w5X00 add device tree support

The w5X00 chip provides an SPI to Ethernet inteface. This patch allows
platform devices to be defined through the device tree.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>