dt-bindings: net: wiznet: add w5x00 support

Add bindings for Wiznet's w5x00 series of SPI interfaced Ethernet chips.

Based on the bindings for microchip,enc28j60.

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