refactor: move `inc/hf/arch/plat/ffa` to `inc/hf/ffa`

Change-Id: I05c2cd1aa3753cd60021015c4582831eb03db3c9
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa/hypervisor/direct_messaging.c b/src/ffa/hypervisor/direct_messaging.c
index e9c0562..2b78f4b 100644
--- a/src/ffa/hypervisor/direct_messaging.c
+++ b/src/ffa/hypervisor/direct_messaging.c
@@ -6,7 +6,7 @@
  * https://opensource.org/licenses/BSD-3-Clause.
  */
 
-#include "hf/arch/plat/ffa/direct_messaging.h"
+#include "hf/ffa/direct_messaging.h"
 
 #include "hf/arch/other_world.h"