1. Add the function for command descriptor preparation which will
   be used only by BAM DMA and it will form the DMA descriptors
   containing command elements
2. DMA_PREP_CMD flag should be used for forming command DMA
   descriptors
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>