Using the keyword argument cs=nnn in the constructor. The cs1
pin has to be defined in mpconfigboard.h.
Note: Only a few boards have the CS1 pin exposed to the connectors.
The ARCH MIX board exposes the Ethernet Pins at it's connectors. Therefore
the software is configured for using a LAN8720 PHY device. Breakout boards
with the LAN8720 are easily available.