Reorganise PSCI PM handler setup on ARM Standard platforms

This patch does the following reorganization to psci power management (PM)
handler setup for ARM standard platform ports :

1. The mailbox programming required during `plat_setup_psci_ops()` is identical
   for all ARM platforms. Hence the implementation of this API is now moved
   to the common `arm_pm.c` file. Each ARM platform now must define the
   PLAT_ARM_TRUSTED_MAILBOX_BASE macro, which in current platforms is the same
   as ARM_SHARED_RAM_BASE.

2. The PSCI PM handler callback structure, `plat_psci_ops`, must now be
   exported via `plat_arm_psci_pm_ops`. This allows the common implementation
   of `plat_setup_psci_ops()` to return a platform specific `plat_psci_ops`.
   In the case of CSS platforms, a default weak implementation of the same is
   provided in `css_pm.c` which can be overridden by each CSS platform.

3. For CSS platforms, the PSCI PM handlers defined in `css_pm.c` are now
   made library functions and a new header file `css_pm.h` is added to export
   these generic PM handlers. This allows the platform to reuse the
   adequate CSS PM handlers and redefine others which need to be customized
   when overriding the default `plat_arm_psci_pm_ops` in `css_pm.c`.

Change-Id: I277910f609e023ee5d5ff0129a80ecfce4356ede
diff --git a/plat/arm/board/fvp/fvp_pm.c b/plat/arm/board/fvp/fvp_pm.c
index 9d6ab9c..21ad14f 100644
--- a/plat/arm/board/fvp/fvp_pm.c
+++ b/plat/arm/board/fvp/fvp_pm.c
@@ -66,17 +66,6 @@
 #endif
 
 /*******************************************************************************
- * Private FVP function to program the mailbox for a cpu before it is released
- * from reset.
- ******************************************************************************/
-static void fvp_program_mailbox(uintptr_t address)
-{
-	uintptr_t *mailbox = (void *) MBOX_BASE;
-	*mailbox = address;
-	flush_dcache_range((uintptr_t) mailbox, sizeof(*mailbox));
-}
-
-/*******************************************************************************
  * Function which implements the common FVP specific operations to power down a
  * cpu in response to a CPU_OFF or CPU_SUSPEND request.
  ******************************************************************************/
@@ -293,9 +282,10 @@
 }
 
 /*******************************************************************************
- * Export the platform handlers to enable psci to invoke them
+ * Export the platform handlers via plat_arm_psci_pm_ops. The ARM Standard
+ * platform layer will take care of registering the handlers with PSCI.
  ******************************************************************************/
-static const plat_psci_ops_t fvp_plat_psci_ops = {
+const plat_psci_ops_t plat_arm_psci_pm_ops = {
 	.cpu_standby = fvp_cpu_standby,
 	.pwr_domain_on = fvp_pwr_domain_on,
 	.pwr_domain_off = fvp_pwr_domain_off,
@@ -307,16 +297,3 @@
 	.validate_power_state = arm_validate_power_state,
 	.validate_ns_entrypoint = arm_validate_ns_entrypoint
 };
-
-/*******************************************************************************
- * Export the platform specific psci ops & initialize the fvp power controller
- ******************************************************************************/
-int plat_setup_psci_ops(uintptr_t sec_entrypoint,
-				const plat_psci_ops_t **psci_ops)
-{
-	*psci_ops = &fvp_plat_psci_ops;
-
-	/* Program the jump address */
-	fvp_program_mailbox(sec_entrypoint);
-	return 0;
-}