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>