refactor(plat/ffa): extract direct messaging

Extract the direct messaging interface from `plat/ffa.h` to
`plat/ffa/direct_messaging.h`.

Change-Id: Idab65bcea7b3e9975a293fa4d83b2b9a6c90b5e8
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/load.c b/src/load.c
index 9968eb7..fc9007f 100644
--- a/src/load.c
+++ b/src/load.c
@@ -13,6 +13,7 @@
 #include "hf/arch/init.h"
 #include "hf/arch/other_world.h"
 #include "hf/arch/plat/ffa.h"
+#include "hf/arch/plat/ffa/direct_messaging.h"
 #include "hf/arch/vm.h"
 
 #include "hf/api.h"