dmaengine: dw: pass platform data via struct dw_dma_chip

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