Update git submodules

* Update docs/submodules/releng from branch 'master'
  - [yardstick] Add stable/gambia branch

    Change-Id: I2f3b0b150cefb12553740f0d4c3ec111fceb7a5b
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index b38b0df..521e788 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit b38b0dfaf377eaa4dcfa4a6504335e524ee9b1cb
+Subproject commit 521e7886bc8d734e0592035acbd562d59e811c65