dmaengine: dw: keep entire platform data in struct dw_dma

Keep the entire platform data in the struct dw_dma.
It makes the driver a bit cleaner.

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>