dm: test: Add a check that all devices have a dev value

With of-platdata, the driver_info struct is updated with the device
pointer when it is bound. This makes it easy for a device to be found by
its driver info with the device_get_by_driver_info() function.

Add a test that all devices (except the root device) have such an entry.
Fix a bug that the function does not set *devp to NULL on failure, which
the documentation asserts.

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