refactor(context mgmt): add cm_prepare_el3_exit_ns function

As part of the RFC:
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/13651,
this patch adds the 'cm_prepare_el3_exit_ns' function. The function is
a wrapper to 'cm_prepare_el3_exit' function for Non-secure state.

When EL2 sysregs context exists (CTX_INCLUDE_EL2_REGS is
enabled) EL1 and EL2 sysreg values are restored from the context
instead of directly updating the registers.

Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I9b071030576bb05500d54090e2a03b3f125d1653
diff --git a/plat/arm/common/aarch64/execution_state_switch.c b/plat/arm/common/aarch64/execution_state_switch.c
index bed929a..2353e6a 100644
--- a/plat/arm/common/aarch64/execution_state_switch.c
+++ b/plat/arm/common/aarch64/execution_state_switch.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2022, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -162,7 +162,7 @@
 	 * calling EL.
 	 */
 	cm_init_my_context(&ep);
-	cm_prepare_el3_exit(NON_SECURE);
+	cm_prepare_el3_exit_ns();
 
 	/*
 	 * State switch success. The caller of SMC wouldn't see the SMC