We pass struct dw_dma_chip to dw_dma_probe() anyway, thus we may use it to
pass a platform data as well.

While here, constify the source of the platform data.
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>