of/flattree: Refactor unflatten_dt_node

unflatten_dt_node is a helper function that does most of the work to
convert a device tree blob into tree of device nodes.  This code
now uses a passed-in blob instead of using the single boot-time blob,
allowing it to be called in more contexts.

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>