Use the correct dtc.git on kernel.org

Commit 3e4b099 "Move dtc from jdl.com to kernel.org" used the wrong
instance of dtc.git on kernel.org. Update the manifest to use the
correct one.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>