refactor(interrupts): cleanup secure interrupt handling code

This patch ensures most of the secure interrupt handling is limited
to the plat/ffa/spmc.c module.

Change-Id: I0a58c709bca56e5fa9c97983e2f5270052be3a8f
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/src/arch/fake/hypervisor/ffa.c b/src/arch/fake/hypervisor/ffa.c
index 9300963..efd270b 100644
--- a/src/arch/fake/hypervisor/ffa.c
+++ b/src/arch/fake/hypervisor/ffa.c
@@ -333,3 +333,12 @@
 {
 	return false;
 }
+bool plat_ffa_msg_wait_prepare(struct vcpu *current, struct vcpu **next,
+			       struct ffa_value *ret_args)
+{
+	(void)current;
+	(void)next;
+	(void)ret_args;
+
+	return false;
+}