Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - ixia: add 1024 and 1280 byte packet size to Ixia ipv4 traffic profile

    This brings it to parity with its non-Ixia counterpart (ipv4_throughput.yaml).

    Change-Id: I4c91d1ae507c7b5e0a23dcf49606b16891b400bd
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 923aa17..6ed2f67 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 923aa1738cbf5a8053e77e00937fc26f2142f6a3
+Subproject commit 6ed2f6721d59b4c52de5d78cef19014a29c7407a