Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - update release notes for Euphrates 5.1.0

    Change-Id: I11b248882dd314baebd3ab4e7e676a1f3e0b50cd
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 23de47c..9d0f993 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 23de47c859f84b7616d8345e96852bde83cd5ab5
+Subproject commit 9d0f99349d2ea4499670708aacb1569cad0389aa