Implement late binding for runtime hooks

At present SPD power management hooks and BL3-2 entry are implemented
using weak references. This would have the handlers bound and registered
with the core framework at build time, but leaves them dangling if a
service fails to initialize at runtime.

This patch replaces implementation by requiring runtime handlers to
register power management and deferred initialization hooks with the
core framework at runtime. The runtime services are to register the
hooks only as the last step, after having all states successfully
initialized.

Change-Id: Ibe788a2a381ef39aec1d4af5ba02376e67269782
diff --git a/services/psci/psci_private.h b/services/psci/psci_private.h
index 351cbe8..4f47ca5 100644
--- a/services/psci/psci_private.h
+++ b/services/psci/psci_private.h
@@ -96,10 +96,9 @@
 extern afflvl_power_on_finisher psci_afflvl_sus_finish_handlers[];
 
 /*******************************************************************************
- * Weak declarations to allow PSCI to cope on a system where the Secure Payload
- * Dispatcher is missing. An SPD will define this structure when present.
+ * SPD's power management hooks registered with PSCI
  ******************************************************************************/
-extern const spd_pm_ops spd_pm;
+extern const spd_pm_ops *psci_spd_pm;
 
 /*******************************************************************************
  * Function prototypes