First pass at DT-probed mmp-camera

[Daniel Drake]

In the end it turns out it does make sense for the sensor (accessible
under /camera-i2c) and the camera (/camera) to be separate.
But Mitch identified the optimal DT layout where the sensor is defined
in the DT at /camera-i2c/image-sensor, which causes i2c-gpio to probe
the ov7670 driver directly.

Pushed to arm-3.5 as f90ea58b3b94fa1597a548817d3c78de15ea8219 and the
OFW bits are in OFW svn.
More info in the commit message.

It's working on XO-1.75 with ov7670. I'm not sure what the status of
the isp island XO-4 code is; once that is done, this code should
hopefully start working there.

Daniel