Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Danube 3.1 release notes

    Change-Id: I7032dd2348dd7212484a4a3cbd40e1d7698c8668
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
    (cherry picked from commit 9449bbf4608d74da8cf61cb4178c844a0095f78f)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index cce6fa7..d553d66 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit cce6fa7f1bca039bedc7484aff6f7862d9074053
+Subproject commit d553d668cfcb24befeeb3464b6d0b5d328c45a2d