dtc: Revert unintentional submodule downgrade from commit c2cabb34

Commit c2cabb34 inadvertently downgraded the 'dtc' submodule,
undoing the increments added in earlier commits. Revert this,
returning the submodule state to where we should be.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>