Allow to choose the chip select pin on ENC28J60.

Nanode uses pin 8 as CS for ethernet module, but not all the ENC28J60
boards uses the same pin.

I've got a Nano Ethernet Shield v1.0 from DK Electronics and it uses pin 10 as CS.

If you like I could add an additional commit with an example sketch working without `NanodeUIO` (not usable due to the lack of MAC EPROM).