avr32: use dw_dmac autoconfiguration

Instead of hardcoding a platform data for dw_dmac let's use it's own
autoconfiguration feature. Thus, remove hardcoded values.

Tested on ATNGW100.

Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>