dtoc: Check that a parent is not missing

With of-platdata-inst we want to set up a reference to each devices'
parent device, if there is one. If we find that the device has a parent
(i.e. is not a root node) but it is not in the list of devices being
written, then we cannot create the reference.

Report an error in this case, since it indicates that the parent node
is either missing a compatible string, is disabled, or perhaps does not
have any properties because it was not tagged for SPL.

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