Add optional bl1_plat_prepare_exit() API

This patch adds an optional API to the platform port:

    void bl1_plat_prepare_exit(void);

This function is called prior to exiting BL1 in response to the
RUN_IMAGE_SMC request raised by BL2. It should be used to perform
platform specific clean up or bookkeeping operations before
transferring control to the next image.

A weak empty definition of this function has been provided to
preserve platform backwards compatibility.

Change-Id: Iec09697de5c449ae84601403795cdb6aca166ba1

bl1/aarch64/bl1_exceptions.S

@@ -207,6 +207,8 @@ func smc_handler64
 	bl	disable_mmu_icache_el3
 	tlbi	alle3
 
+	bl	bl1_plat_prepare_exit
+
 	ldp	x6, x7, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x30)]
 	ldp	x4, x5, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x20)]
 	ldp	x2, x3, [x20, #(ENTRY_POINT_INFO_ARGS_OFFSET + 0x10)]

docs/porting-guide.md

@@ -788,6 +788,17 @@ the entry point set by loader and also set the security state and SPSR which
 represents the entry point system state for BL2.
 
 
+### Function : bl1_plat_prepare_exit() [optional]
+
+    Argument : void
+    Return   : void
+
+This function is called prior to exiting BL1 in response to the `RUN_IMAGE_SMC`
+request raised by BL2. It should be used to perform platform specific clean up
+or bookkeeping operations before transferring control to the next image. This
+function runs with MMU disabled.
+
+
 3.2 Boot Loader Stage 2 (BL2)
 -----------------------------
 

plat/common/aarch64/platform_helpers.S

@@ -37,6 +37,7 @@
 	.weak	plat_crash_console_putc
 	.weak	plat_reset_handler
 	.weak	plat_disable_acp
+	.weak	bl1_plat_prepare_exit
 
 #if !ENABLE_PLAT_COMPAT
 	.globl	platform_get_core_pos
@@ -111,3 +112,12 @@ endfunc plat_reset_handler
 func plat_disable_acp
 	ret
 endfunc plat_disable_acp
+
+	/* -----------------------------------------------------
+	 * void bl1_plat_prepare_exit(void);
+	 * Called before exiting BL1. Default: do nothing
+	 * -----------------------------------------------------
+	 */
+func bl1_plat_prepare_exit
+	ret
+endfunc bl1_plat_prepare_exit