refactor: factorize SPMC's NWd VM view destruction

SPMC keeps a view of NWd VMs structure for mailbox and bitmap
handling. Factorize VM destruction code to be shared between
RXTX_UNMAP and bitmap destroy use-cases.

Change-Id: I72ebf5cfc011d5c660da244b2908142ca0568407
Signed-off-by: Federico Recanati <federico.recanati@arm.com>
diff --git a/src/arch/fake/hypervisor/ffa.c b/src/arch/fake/hypervisor/ffa.c
index 830544a..2f54c71 100644
--- a/src/arch/fake/hypervisor/ffa.c
+++ b/src/arch/fake/hypervisor/ffa.c
@@ -79,6 +79,11 @@
 	return false;
 }
 
+void plat_ffa_vm_destroy(struct vm_locked to_destroy_locked)
+{
+	(void)to_destroy_locked;
+}
+
 void plat_ffa_rxtx_unmap_forward(ffa_vm_id_t id)
 {
 	(void)id;