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/hf_ipi.c b/src/hf_ipi.c
index 3aa58e6..86f18c1 100644
--- a/src/hf_ipi.c
+++ b/src/hf_ipi.c
@@ -8,7 +8,7 @@
 
 #include "hf/hf_ipi.h"
 
-#include "hf/arch/plat/ffa.h"
+#include "hf/arch/plat/ffa/notifications.h"
 
 #include "hf/cpu.h"
 #include "hf/plat/interrupts.h"