dma: dw: allow shared interrupts

In the PC world is quite possible that devices are sharing the same interrupt
line. The patch prepares dw_dmac driver to such cases.

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