powerpc/powernv: Add opal_notifier_unregister() and export to modules

opal_notifier_register() is missing a pending "unregister" variant
and should be exposed to modules.

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/include/asm/opal.h

···
 				   int depth, void *data);
 
 extern int opal_notifier_register(struct notifier_block *nb);
+extern int opal_notifier_unregister(struct notifier_block *nb);
+
 extern int opal_message_notifier_register(enum OpalMessageType msg_type,
 						struct notifier_block *nb);
 extern void opal_notifier_enable(void);

arch/powerpc/platforms/powernv/opal.c

···
 	atomic_notifier_chain_register(&opal_notifier_head, nb);
 	return 0;
 }
+EXPORT_SYMBOL_GPL(opal_notifier_register);
+
+int opal_notifier_unregister(struct notifier_block *nb)
+{
+	if (!nb) {
+		pr_warning("%s: Invalid argument (%p)\n",
+			   __func__, nb);
+		return -EINVAL;
+	}
+
+	atomic_notifier_chain_unregister(&opal_notifier_head, nb);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(opal_notifier_unregister);
 
 static void opal_do_notifier(uint64_t events)
 {