Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - [release] Yardstick Fraser 6.0.0 Release notes

    JIRA: YARDSTICK-1129

    Change-Id: I5adb2e9547c58b90d3eafb0e2b90015ce5472cfa
    Signed-off-by: rexlee8776 <limingjiang@huawei.com>
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
    (cherry picked from commit a324c1b922ae2598dbe25e9b2885620ef09f4901)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index bd2064d..a4c8f2a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit bd2064d2862e2a50566394eba9ce15a0d1d8db40
+Subproject commit a4c8f2a99f56dd4c9fbac4021706aa9186d23ed8