feat(indirect message): add RX_ACQUIRE ABI

In FF-A v1.1 both Hypervisor and SPMC are producers of VM's RX buffer,
and they could contend for the same buffer.
SPMC owns VM's RX buffer after it's mapped in its translation regime.
This ABI should be used by the Hypervisor to get the ownership of a
VM's RX buffer.
This patch covers two uses cases: indirect messages (through v1.1
FFA_MSG_SEND2) and FFA_PARTITION_INFO_GET. Memory sharing interfaces
should make use of RX_ACQUIRE when they will be updated to FF-A v1.1.

Change-Id: Id33cbc7e500adf55316783a48e7acfcb52b9e305
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 9eb3686..2537dc0 100644
--- a/src/arch/fake/hypervisor/ffa.c
+++ b/src/arch/fake/hypervisor/ffa.c
@@ -99,6 +99,15 @@
 	return false;
 }
 
+bool plat_ffa_acquire_receiver_rx(struct vm_locked to_locked,
+				  struct ffa_value *ret)
+{
+	(void)to_locked;
+	(void)ret;
+
+	return false;
+}
+
 bool plat_ffa_is_indirect_msg_supported(struct vm_locked sender_locked,
 					struct vm_locked receiver_locked)
 {