Flush dcache when storing timestamp

On DynamIQ CPU FVPs, stats test cases are failing when
hardware-assisted coherency is enabled due to a corrupt
timestamp value. Investigation of the issue indicates that
on these models the timestamp value is stored in cache
instead of memory. This patch flushes the dcache when the
timestamp is stored to make sure it is stored in memory.

Change-Id: I05cd54ba5991a5a96dd07f1e08b5212273201411
Signed-off-by: Zelalem <zelalem.aweke@arm.com>

plat/common/plat_psci_common.c

 /*
- * Copyright (c) 2016-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2020, ARM Limited and Contributors. All rights reserved.
  * Copyright (c) 2020, NVIDIA Corporation. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
@@ -63,7 +63,6 @@ static u_register_t calc_stat_residency(unsigned long long pwrupts,
 
 /*
  * Capture timestamp before entering a low power state.
- * No cache maintenance is required when capturing the timestamp.
  * Cache maintenance may be needed when reading these timestamps.
  */
 void plat_psci_stat_accounting_start(
@@ -71,12 +70,11 @@ void plat_psci_stat_accounting_start(
 {
 	assert(state_info != NULL);
 	PMF_CAPTURE_TIMESTAMP(psci_svc, PSCI_STAT_ID_ENTER_LOW_PWR,
-		PMF_NO_CACHE_MAINT);
+		PMF_CACHE_MAINT);
 }
 
 /*
  * Capture timestamp after exiting a low power state.
- * No cache maintenance is required when capturing the timestamp.
  * Cache maintenance may be needed when reading these timestamps.
  */
 void plat_psci_stat_accounting_stop(
@@ -84,7 +82,7 @@ void plat_psci_stat_accounting_stop(
 {
 	assert(state_info != NULL);
 	PMF_CAPTURE_TIMESTAMP(psci_svc, PSCI_STAT_ID_EXIT_LOW_PWR,
-		PMF_NO_CACHE_MAINT);
+		PMF_CACHE_MAINT);
 }
 
 /*