Hi.  I've found some sources for the SONIC ethernet driver used on
upper-level Quadra/Centris models. I don't know if these are the best
sources to using to get onboard ethernet working on the Centris/Quadra
line....
It can be found at ftp.openbsd.org/mirrors5/OpenBSD/src/sys/arch/wgrisc/dev/

The files are named if_sn.c and if_sn.h respectively.

Hope this'll hope the porting effort.

Steve