dtc: create tool to diff device trees

Create script to diff device trees.

The device tree can be in any of the forms recognized by the dtc compiler:
  - source
  - binary blob
  - file system tree (from /proc/devicetree)

If the device tree is a source file, then it is pre-processed in the
same way as it would be when built in the linux kernel source tree
before diffing.

Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Rob Herring <robh@kernel.org>