FDT: Fix DTC repository references

The Device Tree Compiler (DTC) used to have its master
repository located on jdl.com.  While it is still there,
its official, new, shiny location is on kernel.org here:

    git://git.kernel.org/pub/scm/utils/dtc/dtc.git



Update a few references to point there instead.

Signed-off-by: Jon Loeliger <jdl@jdl.com>
Acked-by: Simon Glass <sjg@chromium.org>