Suppress openamp warnings

Added '-Wno-cast-align' and '-Wno-unused-but-set-variable' options to
suppress openamp related warnings

Change-Id: I67c82f0639a580fc4d8d49ef12e45ead36d4fc82
diff --git a/openamp/CMakeLists.txt b/openamp/CMakeLists.txt
index 62abbdd..0a756c8 100644
--- a/openamp/CMakeLists.txt
+++ b/openamp/CMakeLists.txt
@@ -55,7 +55,9 @@
         $<$<STREQUAL:${PROJECT_SYSTEM},freertos>:freertos_kernel>)
 
     target_compile_options(openamp-${PROJECT_SYSTEM} PRIVATE
+        -Wno-cast-align
         -Wno-unknown-pragmas
+        -Wno-unused-but-set-variable
     )
 
     # Generate libmetal headers