wip: temporary bump of gitmodule for demonstration purposes.

This is part of draft pull request. Do not merge.
provide gitmodule reference from sipke's forked repository to show
the openamp-system-reference readme integrations into readthedocs.
will delete once reviewed and submit PR as not draft.

Signed-off-by: Sipke Vriend <sipke@direktembedded.com>

Author: Sipke Vriend

.gitmodules

@@ -9,7 +9,7 @@
 	url = https://github.com/devicetree-org/lopper.git
 [submodule "openamp-system-reference"]
 	path = openamp-system-reference
-	url = https://github.com/OpenAMP/openamp-system-reference.git
+	url = https://github.com/sipke/openamp-system-reference.git
 [submodule "libmetal"]
 	path = libmetal
 	url = https://github.com/OpenAMP/libmetal.git