dtoc: Drop dm_populate_phandle_data()

This has not been needed since parent information was added and we started
using indicies for references to other drivers instead of pointers. It was
kept around in the expectation that it might be needed later.

However with the latest updates, it doesn't seem likely that we'll need
this in the foreseeable future.

Drop dm_populate_phandle_data() from dtoc and driver model.

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