dmaengine: Mark the struct dma_slave_config direction field deprecated

The direction passed to the device_prep_slave_sg, device_prep_dma_cyclic
or device_prep_interleaved_dma (through struct dma_interleaved_template)
should be used instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>