dtc: Revert unintentional submodule downgrade from commit 077dd742

Commit 077dd742 inadvertently downgraded the 'dtc' submodule,
undoing the increment added in commit 6e85fce0. Revert this,
returning the submodule state to where we should be.

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