dm: core: Complete phandle implementation using the other FDT

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>