rtc: at91sam9: Fix missing spin_lock_init()

The driver allocates the spinlock but not initialize it.
Use spin_lock_init() on it to initialize it correctly.

This is detected by Coccinelle semantic patch.
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-at91sam9.c

@@ -375,6 +375,7 @@ static int at91_rtc_probe(struct platform_device *pdev)
 	if (!rtc)
 		return -ENOMEM;
 
+	spin_lock_init(&rtc->lock);
 	rtc->irq = irq;
 
 	/* platform setup code should have handled this; sigh */