dmaengine: dw: split dma-dw.h to platform and private parts

The introduced include/linux/dma/dw.h is going to contain the private
extensions and structures which are shared for dw_dmac users in the kernel.
Meanwhile include/linux/platform_data/dma-dw.h keeps only platform related data
types and definitions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>