mailbox: Make startup and shutdown ops optional

Some mailbox hardware doesn't have to perform any additional operations
on startup of shutdown, so make these optional.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox.c

@@ -355,12 +355,15 @@ struct mbox_chan *mbox_request_channel(struct mbox_client *cl, int index)
 
 	spin_unlock_irqrestore(&chan->lock, flags);
 
+	if (chan->mbox->ops->startup) {
 		ret = chan->mbox->ops->startup(chan);
+
 		if (ret) {
 			dev_err(dev, "Unable to startup the chan (%d)\n", ret);
 			mbox_free_channel(chan);
 			chan = ERR_PTR(ret);
 		}
+	}
 
 	mutex_unlock(&con_mutex);
 	return chan;
@@ -408,6 +411,7 @@ void mbox_free_channel(struct mbox_chan *chan)
 	if (!chan || !chan->cl)
 		return;
 
+	if (chan->mbox->ops->shutdown)
 		chan->mbox->ops->shutdown(chan);
 
 	/* The queued TX requests are simply aborted, no callbacks are made */