of/flattree: Refactor unflatten_device_tree and add fdt_unflatten_tree

unflatten_device_tree has two dependencies on things that happen
during boot time.  Firstly, it references the initial device tree
directly. Secondly, it allocates memory using the early boot
allocator.  This patch factors out these dependencies and uses
the new __unflatten_device_tree function to implement a driver-visible
fdt_unflatten_tree function, which can be used to unflatten a
blob after boot time.

V2:
- remove extra __va() call
- make dt_alloc functions return void *.  This doesn't fix the general
  strangeness in this code that constantly casts back and forth between
  unsigned long and __be32 *

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