refactor(plat/ffa): extract indirect messaging

Extract the indirect messaging interface from `plat/ffa.h` to
`plat/ffa/indirect_messaging.h`.

Change-Id: Idca4d0af7640ab61cb8323688aca6c97cf7f0463
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa_memory.c b/src/ffa_memory.c
index 056fd86..cb216e2 100644
--- a/src/ffa_memory.c
+++ b/src/ffa_memory.c
@@ -12,6 +12,7 @@
 #include "hf/arch/mm.h"
 #include "hf/arch/other_world.h"
 #include "hf/arch/plat/ffa.h"
+#include "hf/arch/plat/ffa/indirect_messaging.h"
 
 #include "hf/addr.h"
 #include "hf/api.h"