mtd: nand: qcom: add BAM DMA descriptor handling

1. prepare_bam_async_desc is the function which will call
   all the DMA API’s. It will fetch the outstanding scatter gather
   list for passed channel and will do the DMA descriptor formation.
   The DMA flag is dependent upon the type of channel.

2. For ADM DMA, the descriptor is being formed for every DMA
   request so its sgl count will be always 1 while in BAM DMA, the
   clubbing of descriptor is being done to increase throughput.

3. ADM DMA uses only one channel while in BAM DMA, data descriptors
   will be submitted to tx channel (for write) or rx channel
   (for read) and all the registers read/write descriptors in
   command channel.

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>