Add support for SMCCC_VERSION in PSCI features

On some platforms it may be necessary to discover the SMCCC version
via a PSCI features call.

Change-Id: I95281ac2263ca9aefda1809eb03464fbdb8ac24d
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>

lib/psci/psci_main.c

 /*
- * Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include <arch.h>
 #include <arch_helpers.h>
+#include <arm_arch_svc.h>
 #include <assert.h>
 #include <debug.h>
 #include <platform.h>
@@ -322,6 +323,9 @@ int psci_features(unsigned int psci_fid)
 {
 	unsigned int local_caps = psci_caps;
 
+	if (psci_fid == SMCCC_VERSION)
+		return PSCI_E_SUCCESS;
+
 	/* Check if it is a 64 bit function */
 	if (((psci_fid >> FUNCID_CC_SHIFT) & FUNCID_CC_MASK) == SMC_64)
 		local_caps &= PSCI_CAP_64BIT_MASK;