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/bl31/bl31_main.c b/bl31/bl31_main.c
index 797093e..2a3d838 100644
--- a/bl31/bl31_main.c
+++ b/bl31/bl31_main.c
@@ -259,7 +259,16 @@
 		(image_type == SECURE) ? "secure" : "normal");
 	print_entry_point_info(next_image_info);
 	cm_init_my_context(next_image_info);
-	cm_prepare_el3_exit(image_type);
+
+	/*
+	* If we are entering the Non-secure world, use
+	* 'cm_prepare_el3_exit_ns' to exit.
+	*/
+	if (image_type == NON_SECURE) {
+		cm_prepare_el3_exit_ns();
+	} else {
+		cm_prepare_el3_exit(image_type);
+	}
 }
 
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