Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Refine documentation

    Change-Id: I0d3387a520e3decf51bb0f7db53996f148a611d0
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit 2347f3977823bfd0c4b1fa832d9046b8a625596b)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index b2a7981..6c0f830 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit b2a79813f57a94b6565fa8f967d10d295010dfff
+Subproject commit 6c0f8306f7bed99905af7a21fc5171642d3c4827