dmaengine: dw: enable runtime PM

On runtime PM aware platforms the DMA have to manage its own power state. This
patch enables runtime PM support and applies necessary calls wherever it's
needed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Tested-by: Scott Ashcroft <scott.ashcroft@talk21.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>