dma: of-dma: protect list write operation by spin_lock

It's possible to have an inconsistency in the list due to unprotected operation
on it. The patch adds a proper locking on the list operation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/of-dma.c

@@ -118,7 +118,9 @@ int of_dma_controller_register(struct device_node *np,
 	ofdma->use_count = 0;
 
 	/* Now queue of_dma controller structure in list */
+	spin_lock(&of_dma_lock);
 	list_add_tail(&ofdma->of_dma_controllers, &of_dma_list);
+	spin_unlock(&of_dma_lock);
 
 	return 0;
 }