of/flattree: Update dtc to current mainline.

Pull in recent changes from the main dtc repository. These changes
primarily allow multiple device trees to be declared which are merged
by dtc. This feature allows us to include a basic dts file and then
provide more information for the specific system through the merging
functionality.

Changes pulled from git://git.jdl.com/software/dtc.git
commit id: 37c0b6a0, "dtc: Add code to make diffing trees easier"

Signed-off-by: John Bonesio <bones@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>