feat(indirect message): add FFA_RXTX_UNMAP forwarding to SPMC

FFA_RXTX_UNMAP has to be forwarded to SPMC to allow it to unmap VM's
buffers, mapped through FFA_RXTX_MAP forwarding.

Change-Id: Ieb8dc04cc61b197023da0f5aa3612cf5a649d9c8
Signed-off-by: Federico Recanati <federico.recanati@arm.com>
diff --git a/inc/hf/arch/plat/ffa.h b/inc/hf/arch/plat/ffa.h
index 8c8e67d..fe1ad4e 100644
--- a/inc/hf/arch/plat/ffa.h
+++ b/inc/hf/arch/plat/ffa.h
@@ -103,6 +103,8 @@
 
 void plat_ffa_rxtx_map_forward(struct vm_locked vm_locked);
 
+void plat_ffa_rxtx_unmap_forward(ffa_vm_id_t id);
+
 /**
  * Checks whether managed exit is supported by given SP.
  */