Update git submodules

Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - Merge "trex: Update T-Rex to v2.38"
  - trex: Update T-Rex to v2.38

    Default API version of T-Rex was changed to v2.38. T-Rex related
    documentation was updated to inform user, that the same version
    of T-Rex must be used for both server and client.

    JIRA: VSPERF-564

    Change-Id: Ib1bedde683efafa84a6069b55daa0507700dbb30
    Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Richard Elias <richardx.elias@intel.com>