mtd: nand: qcom: support for command descriptor formation

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>