We need to be able to look up phandles in any FDT, not just the control
FDT. Use the 'other' FDT to test this, with a helper function which gets
this as an oftree that can then we used as needed.
Add a few more tests and some comments at the top of the file, to explain
what is going on.
Signed-off-by: Simon Glass <sjg@chromium.org>