Build: Improve NS mailbox config flag setting

Support a flag TFM_MULTI_CORE_NS_OS to control NS integration.

When integrating TF-M with NS OS on dual-cpu platforms, select
TFM_MULTI_CORE_NS_OS in NS build to enable NS OS support in NS
mailbox.
When integrating TF-M with NS bare metal environment, disable
TFM_MULTI_CORE_NS_OS and NS mailbox relies on looping and requires
no support from NS OS.

TFM_MULTI_CORE_MULTI_CLIENT_CALL is removed.
NS mailbox build can get rid of the dependency on the configuration
of TFM_MULTI_CORE_MULTI_CLIENT_CALL in S mailbox build.

NUM_MAILBOX_QUEUE_SLOT directly indicates whether platform and SPE
supports multiple NS PSA Client calls.

Change-Id: I937b2afdb88cccc22a20617d2ab36bcc1b939b05
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/platform/CMakeLists.txt b/platform/CMakeLists.txt
index 5e493b1..93f5068 100755
--- a/platform/CMakeLists.txt
+++ b/platform/CMakeLists.txt
@@ -1,5 +1,5 @@
 #-------------------------------------------------------------------------------
-# Copyright (c) 2020, Arm Limited. All rights reserved.
+# Copyright (c) 2020-2021, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -96,7 +96,6 @@
     PRIVATE
         $<$<BOOL:${TFM_IRQ_TEST}>:TFM_ENABLE_IRQ_TEST>
         $<$<BOOL:${TFM_PERIPH_ACCESS_TEST}>:TFM_ENABLE_PERIPH_ACCESS_TEST>
-        $<$<BOOL:${TFM_MULTI_CORE_MULTI_CLIENT_CALL}>:TFM_MULTI_CORE_MULTI_CLIENT_CALL>
 )
 
 #========================= Platform BL2 =======================================#