dmaengine: dw: introduce dw_dma_on() helper

As an opposite to dw_dma_off() let's introduce dw_dma_on() helper. It will be
useful later as well.

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