spd: trusty: save context starting from the stack end

This patch uses the stack end to start saving the CPU context
during world switch. The previous logic, used the stack start
to save the context, thus overwriting the other members of the
context.
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

services/spd/trusty/trusty.c

 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -50,6 +50,7 @@
 
 struct trusty_stack {
 	uint8_t space[PLATFORM_STACK_SIZE] __aligned(16);
+	uint32_t end;
 };
 
 struct trusty_cpu_ctx {
@@ -326,7 +327,7 @@ static int32_t trusty_init(void)
 	cm_set_next_eret_context(SECURE);
 
 	ctx->saved_security_state = ~0; /* initial saved state is invalid */
-	trusty_init_context_stack(&ctx->saved_sp, &ctx->secure_stack);
+	trusty_init_context_stack(&ctx->saved_sp, &ctx->secure_stack.end);
 
 	trusty_context_switch_helper(&ctx->saved_sp, &zero_args);