refactor(plat/ffa): extract VM

Extract the VM interface from `plat/ffa.h` to `plat/ffa/vm.h`.

Change-Id: I3d1d8811a61c5c3c712e9370371b5ac896033b15
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/api.c b/src/api.c
index 527a0d8..07d223f 100644
--- a/src/api.c
+++ b/src/api.c
@@ -16,6 +16,7 @@
 #include "hf/arch/plat/ffa.h"
 #include "hf/arch/plat/ffa/direct_messaging.h"
 #include "hf/arch/plat/ffa/indirect_messaging.h"
+#include "hf/arch/plat/ffa/vm.h"
 #include "hf/arch/timer.h"
 #include "hf/arch/vm.h"