Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Enable iommu support in grub"
  - Enable iommu support in grub

    JIRA: YARDSTICK-895
    It's required for sriov standalone yardstick tests.
    This part of commit message will be deleted:
    This code will be enabled when https://gerrit.opnfv.org/gerrit/#/c/47957/ will be merged,
    because it requires host reboot.

    Change-Id: I0ec9cbf90a9d83d54e545fcf421b8baf206601a4
    Signed-off-by: Jan Malanik <janx.malanik@intel.com>
    Signed-off-by: Malanik Jan <janx.malanik@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 12aaf2f..7f8f31c 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 12aaf2fd5a8511570fa20d9a5ca3643113475f24
+Subproject commit 7f8f31cf68dbecaf521348fccf540cbe0e7c72bd