TODO for slave dma
  
  1. Move remaining drivers to use new slave interface
  2. Remove old slave pointer machansim
  3. Make issue_pending to start the transaction in below drivers
  	- mpc512x_dma
  	- imx-dma
  	- imx-sdma
  	- mxs-dma.c
  	- intel_mid_dma
  4. Check other subsystems for dma drivers and merge/move to dmaengine
  5. Remove dma_slave_config's dma direction.