dtc: Update to version 1.6.1

The dtc submodule is currently pointing to non-release commit. It's nicer
if submodules point to release versions instead and since dtc 1.6.1 is
available now, let's update to that version.

Message-Id: <[email protected]>
Acked-by: Greg Kurz <[email protected]>
Acked-by: David Gibson <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>

Author: huth