refactor(plat/ffa): extract notifications

Extract the notifications interface from `plat/ffa.h` to
`plat/ffa/notifications.h`.

Change-Id: Iebfb2c1bd5562a6f8dc78ce45ba4acd14075fd45
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/arch/aarch64/hypervisor/handler.c b/src/arch/aarch64/hypervisor/handler.c
index 89274cd..601d3ed 100644
--- a/src/arch/aarch64/hypervisor/handler.c
+++ b/src/arch/aarch64/hypervisor/handler.c
@@ -16,6 +16,7 @@
 #include "hf/arch/mmu.h"
 #include "hf/arch/plat/ffa.h"
 #include "hf/arch/plat/ffa/indirect_messaging.h"
+#include "hf/arch/plat/ffa/notifications.h"
 #include "hf/arch/plat/ffa/vm.h"
 #include "hf/arch/plat/smc.h"
 #include "hf/arch/timer.h"