dtoc: Generate device instances

Add support for generating a file containing udevice instances. This
avoids the need to create these at run time.

Update a test uclass to include a 'per_device_plat_auto' member, to
increase test coverage.

Add another tab to the driver_info output so it lines up nicely like the
device-instance output.

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