of: unittest: unflatten device tree on UML when testing

UML supports enabling OF, and is useful for running the device tree
tests, so add support for unflattening device tree blobs so we can
actually use it.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>