ArmPkg/IndustryStandard: Add barebones FF-A header

This patch adds a rudimentary header file with defines for FF-A ABIs
that will be used as the transport between S-EL0 and the SPM

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmPkg/Include/IndustryStandard/ArmFfaSvc.h

@@ -0,0 +1,23 @@
+/** @file
+  Header file for FF-A ABI's that will be used for
+  communication between S-EL0 and the Secure Partition
+  Manager(SPM)
+
+  Copyright (c) 2020, ARM Limited. All rights reserved.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+  @par Revision Reference:
+    - FF-A Version 1.0
+
+
+**/
+
+#ifndef ARM_FFA_SVC_H_
+#define ARM_FFA_SVC_H_
+
+#define ARM_SVC_ID_FFA_VERSION_AARCH32                  0x84000063
+#define ARM_SVC_ID_FFA_MSG_SEND_DIRECT_REQ_AARCH64      0xC400006F
+#define ARM_SVC_ID_FFA_MSG_SEND_DIRECT_RESP_AARCH64     0xC4000070
+
+#endif // ARM_FFA_SVC_H_