refactor(ff-a): interrupts

Some functions from `src/ffa/{hypervisor,spmc}.c` were missed in the
refactoring.
Move them to `src/ffa/{hypervisor,spmc}/interrupts.c`.

Change-Id: I97b80ce613964e43f58f52e8a9cff57cf63fedff
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa/hypervisor/interrupts.c b/src/ffa/hypervisor/interrupts.c
index 21d89ed..9bae658 100644
--- a/src/ffa/hypervisor/interrupts.c
+++ b/src/ffa/hypervisor/interrupts.c
@@ -56,3 +56,14 @@
 {
 	return api_interrupt_get(current_locked);
 }
+
+bool ffa_interrupts_intercept_call(struct vcpu_locked current_locked,
+				   struct vcpu_locked next_locked,
+				   struct ffa_value *signal_interrupt)
+{
+	(void)current_locked;
+	(void)next_locked;
+	(void)signal_interrupt;
+
+	return false;
+}