net: w5100: support SPI interface mode

This adds new w5100-spi driver which shares the bus interface
independent code with existing w5100 driver.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>