of: fdt: mark unflattened tree as detached

The tree returned from of_fdt_unflatten_tree cannot be attached to the
live tree because it is not marked as detached so mark it as such. The
dt resolver checks the flag and refuses to process the tree otherwise.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>