Fix PSCI STAT time stamp collection

This patch includes various fixes for PSCI STAT functionality
relating to timestamp collection:

1. The PSCI stat accounting for retention states for higher level
power domains were done outside the locks which could lead to
spurious values in some race conditions. This is moved inside
the locks. Also, the call to start the stat accounting was redundant
which is now removed.

2. The timestamp wrap-around case when calculating residency did
not cater for AArch32. This is now fixed.

3. In the warm boot path, `plat_psci_stat_accounting_stop()` was
getting invoked prior to population of target power states. This
is now corrected.

Change-Id: I851526455304fb74ff0a724f4d5318cd89e19589
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

lib/psci/psci_suspend.c

@@ -37,6 +37,11 @@ static void psci_suspend_to_standby_finisher(unsigned int cpu_idx,
 	 */
 	psci_get_target_local_pwr_states(end_pwrlvl, &state_info);
 
+#if ENABLE_PSCI_STAT
+	plat_psci_stat_accounting_stop(&state_info);
+	psci_stats_update_pwr_up(end_pwrlvl, &state_info);
+#endif
+
 	/*
 	 * Plat. management: Allow the platform to do operations
 	 * on waking up from retention.
@@ -236,10 +241,6 @@ exit:
 	    PMF_NO_CACHE_MAINT);
 #endif
 
-#if ENABLE_PSCI_STAT
-	plat_psci_stat_accounting_start(state_info);
-#endif
-
 	/*
 	 * We will reach here if only retention/standby states have been
 	 * requested at multiple power levels. This means that the cpu
@@ -247,11 +248,6 @@ exit:
 	 */
 	wfi();
 
-#if ENABLE_PSCI_STAT
-	plat_psci_stat_accounting_stop(state_info);
-	psci_stats_update_pwr_up(end_pwrlvl, state_info);
-#endif
-
 #if ENABLE_RUNTIME_INSTRUMENTATION
 	PMF_CAPTURE_TIMESTAMP(rt_instr_svc,
 	    RT_INSTR_EXIT_HW_LOW_PWR,