The gpioiic(4) driver allows to attach I2C busses using GPIO bit- banging. Ported from OpenBSD.Please note that a gpioiic(4) driver, once attached to gpio(4) device, can not be detached because the child device, the iic bus, can not bet detached (if possible I will change that).The diff is also available online at http://www.netbsd.org/~mbalmer/diffs/gpioiic_01.diff
I forgot the list parts. You find an updated diff at http://www.netbsd.org/~mbalmer/diffs/gpioiic_02.diff