dtc: Revert unintentional submodule downgrade from commit c2cabb34220d63f9

Commit c2cabb34220d63f9 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>

diff --git a/dtc b/dtc
index 65cc4d2748a2c2e6f27f1cf39e07a5dbabd80ebf..558cd81bdd432769b59bff01240c44f82cfb1a9d 160000 (submodule)
--- a/dtc
+++ b/dtc
@@ -1 +1 @@
-Subproject commit 65cc4d2748a2c2e6f27f1cf39e07a5dbabd80ebf
+Subproject commit 558cd81bdd432769b59bff01240c44f82cfb1a9d