Update git submodules

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

    Change-Id: I7032dd2348dd7212484a4a3cbd40e1d7698c8668
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index fb86be5..9449bbf 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit fb86be55d52f6cefd4e0de4b857ac76d00a9e681
+Subproject commit 9449bbf4608d74da8cf61cb4178c844a0095f78f