feat(interrupts): preferred managed exit signal

A SP optionally specifies if vIRQ is the preferred choice for
signaling Managed Exit in response to a non secure interrupt.

Moreover, SPMC enables the following virtual maintenenace interrupts
by default for each Secure Partition:
  > Managed Exit Interrupt
  > Notification Pending Interrupt

Change-Id: I17131b42ee6fae2896804e4f952d9a371f237cd5
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 c41169b..0f45e3b 100644
--- a/src/arch/fake/hypervisor/ffa.c
+++ b/src/arch/fake/hypervisor/ffa.c
@@ -465,3 +465,9 @@
 	(void)current;
 	(void)next;
 }
+
+void plat_ffa_enable_virtual_maintenance_interrupts(
+	struct vcpu_locked current_locked)
+{
+	(void)current_locked;
+}