dm: core: Avoid registering an inaccessible tree

dm: core: Avoid registering an inaccessible tree

At present there are various restrictions on the use of livetree:

- It is only available once the tree is unflattened, i.e. after relocation
- It is designed to be used with the control FDT
- It can (in principle) be used with other FDTs, but only if they are
  unflattened first; this is not supported

Add a few checks to make sure that any tree that is created is actually
valid. Otherwise it can be confusing when nodes and properties cannot
actually be accessed.

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