lib/open-amp: Add OpenAMP submodule.

OpenAMP framework provides a standard inter processor communications
infrastructure for RTOS and bare metal environments. There are 3 major
components in OpenAMP: libmetal, remoteproc and RPMsg. libmetal provides
abstraction of the low-level underlying hardware, remoteproc is used for
processor Life Cycle Management (LCM) like loading firmware, starting,
stopping a core etc., and RPMsg is a bus infrastructure that enables Inter
Processor Communications (IPC) between different cores.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

.gitmodules

@@ -59,3 +59,6 @@
 [submodule "lib/protobuf-c"]
 	path = lib/protobuf-c
 	url = https://github.com/protobuf-c/protobuf-c.git
+[submodule "lib/open-amp"]
+	path = lib/open-amp
+	url = https://github.com/OpenAMP/open-amp.git

lib/open-amp

@@ -0,0 +1 @@
+Subproject commit 1904dee18da85400e72b8f55c5c99e872a486573