dmaengine: dw_dmac: Update documentation style comments for dw_dma_platform_data

Documentation style comments were missing for few fields in struct
dw_dma_platform_data. Add these.

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