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/include/psci.h b/include/psci.h
index 3a040f9..922f95f 100644
--- a/include/psci.h
+++ b/include/psci.h
@@ -172,6 +172,7 @@
 		       unsigned long);
 extern void psci_aff_on_finish_entry(void);
 extern void psci_aff_suspend_finish_entry(void);
+extern void psci_register_spd_pm_hook(const spd_pm_ops *);
 #endif /*__ASSEMBLY__*/