DT: Add overlays to configure I2C pins

Lifted from
https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=120938&p=825883
so not claiming this to be my own work.
Adds overlays i2c0-bcm2708 and i2c1-bcm2708 that allow the pin
allocations for i2c-0 and i2c-1 to be changed.