lib/libmetal: Add libmetal submodule.

libmetal provides an abstraction of the underlying hardware, to support
other OpenAMP components.

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

.gitmodules

@@ -62,3 +62,6 @@
 [submodule "lib/open-amp"]
 	path = lib/open-amp
 	url = https://github.com/OpenAMP/open-amp.git
+[submodule "lib/libmetal"]
+	path = lib/libmetal
+	url = https://github.com/OpenAMP/libmetal.git

lib/libmetal

@@ -0,0 +1 @@
+Subproject commit 0cb7d293a7f25394a06847a28d0f0ace9862936e