dmaengine: dw: convert dw_dma_slave to use explicit HS interfaces

Instead of exposing the possibility to set DMA registers CFG_HI and CFG_LO
strict user to provide handshake interfaces explicitly.

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