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.