refactor(psci): unify psci_is_last_on_cpu and psci_is_last_on_cpu_safe

"psci_is_last_on_cpu" and "psci_is_last_on_cpu_safe" modules perform
mostly similar functionalities, verifying whether the current CPU
is the only active core and other cores have been turned off.

However, psci_is_last_on_cpu_safe function differs from the other with:
1. Safe API locks the power domain

This patch removes the section duplicating the functionality
and ensures that "psci_is_last_on_cpu api",is reused in
"psci_is_last_on_cpu_safe" procedure.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: Ie372519e423898d7afa5427cdd77a7f9d3369587
diff --git a/lib/psci/psci_main.c b/lib/psci/psci_main.c
index 52a8b8a..a631f3f 100644
--- a/lib/psci/psci_main.c
+++ b/lib/psci/psci_main.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2022, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -158,7 +158,7 @@
 	entry_point_info_t ep;
 
 	/* Check if the current CPU is the last ON CPU in the system */
-	if (psci_is_last_on_cpu() == 0U)
+	if (!psci_is_last_on_cpu())
 		return PSCI_E_DENIED;
 
 	/* Validate the entry point and get the entry_point_info */