plat/common: move arch-agnostic fallback functions to C file

When we add a new callback, we need to duplicate fallbacks among
plat/common/{aarch32,aarch64}/platform_helpers.S  This is tedious.

I created a new C file, then moved 3 functions:
  plat_error_handler
  bl2_plat_preload_setup
  plat_try_next_boot_source

They are called from C, so I do not see a good reason to implement
them in assembly.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/plat/common/aarch64/platform_helpers.S b/plat/common/aarch64/platform_helpers.S
index 8526752..033a12f 100644
--- a/plat/common/aarch64/platform_helpers.S
+++ b/plat/common/aarch64/platform_helpers.S
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -16,10 +16,7 @@
 	.weak	plat_reset_handler
 	.weak	plat_disable_acp
 	.weak	bl1_plat_prepare_exit
-	.weak	plat_error_handler
 	.weak	plat_panic_handler
-	.weak	bl2_plat_preload_setup
-	.weak	plat_try_next_boot_source
 
 #if !ENABLE_PLAT_COMPAT
 	.globl	platform_get_core_pos
@@ -159,16 +156,6 @@
 endfunc bl1_plat_prepare_exit
 
 	/* -----------------------------------------------------
-	 * void plat_error_handler(int err) __dead2;
-	 * Endless loop by default.
-	 * -----------------------------------------------------
-	 */
-func plat_error_handler
-	wfi
-	b	plat_error_handler
-endfunc plat_error_handler
-
-	/* -----------------------------------------------------
 	 * void plat_panic_handler(void) __dead2;
 	 * Endless loop by default.
 	 * -----------------------------------------------------
@@ -177,22 +164,3 @@
 	wfi
 	b	plat_panic_handler
 endfunc plat_panic_handler
-
-	/* -----------------------------------------------------
-	 * Placeholder function which should be redefined by
-	 * each platfrom.
-	 * -----------------------------------------------------
-	 */
-func bl2_plat_preload_setup
-	ret
-endfunc bl2_plat_preload_setup
-
-	/* -----------------------------------------------------
-	 * Placeholder function which should be redefined by
-	 * each platfrom.
-	 * -----------------------------------------------------
-	 */
-func plat_try_next_boot_source
-	mov	x0, #0
-	ret
-endfunc plat_try_next_boot_source