Update git submodules

* Update docs/submodules/vswitchperf from branch 'stable/danube'
  - Updated index

    Change-Id: I9e180eff666c3ca98eb51d9c958e3b1410ddddd5
    Signed-off-by: Trevor Cooper <trevor.cooper@intel.com>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index 815edb9..8a75da0 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit 815edb966a5c7c1c82083c7e726885d08cf5e241
+Subproject commit 8a75da0f109a6a0ee5b6b347ceab367727f9640c