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/include/lib/el3_runtime/context_mgmt.h b/include/lib/el3_runtime/context_mgmt.h
index 2090687..1a76d8e 100644
--- a/include/lib/el3_runtime/context_mgmt.h
+++ b/include/lib/el3_runtime/context_mgmt.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2022, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -34,6 +34,7 @@
 			      const struct entry_point_info *ep);
 void cm_setup_context(cpu_context_t *ctx, const struct entry_point_info *ep);
 void cm_prepare_el3_exit(uint32_t security_state);
+void cm_prepare_el3_exit_ns(void);
 
 #ifdef __aarch64__
 #if CTX_INCLUDE_EL2_REGS