dmaengine: dw: move private definitions to regs.h

Since we don't allow user to set registers directly through private slave
configuration we may move definitions to the regs.h because they are not used
anywhere except core.c part.

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