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_afflvl_off.c b/services/psci/psci_afflvl_off.c
index 24c212f..3763f6f 100644
--- a/services/psci/psci_afflvl_off.c
+++ b/services/psci/psci_afflvl_off.c
@@ -65,8 +65,8 @@
 	 * to let it do any bookeeping. Assume that the SPD always reports an
 	 * E_DENIED error if SP refuse to power down
 	 */
-	if (spd_pm.svc_off) {
-		rc = spd_pm.svc_off(0);
+	if (psci_spd_pm && psci_spd_pm->svc_off) {
+		rc = psci_spd_pm->svc_off(0);
 		if (rc)
 			return rc;
 	}