Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - Merge "[doc] Add guest OS support matrix"
  - [doc] Add guest OS support matrix

    Update fuel documentation regarding the guest operating systems
    that we can boot in both in x86 and aarch64.

    Change-Id: Iea96fae880755d0a9cc8fa519362ff564f79e391
    Signed-off-by: Charalampos Kominos <Charalampos.Kominos@enea.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 9e9393a..71639a2 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 9e9393ab60ff513f01b9ce045245e6c5808590a8
+Subproject commit 71639a2c6da028aa98b5e324ddeb650ebfb4ead6