dm: core: Set up driver model for OF_PLATDATA_INST

With this we don't need to scan and bind drivers, not even the root
device. We just need to locate the root device that was set up at build
time, then set our root in global_data to point to it.

Update the code to handle this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>