The EBI2 NAND controller directly remaps register read buffer with
dma_map_sg and DMA address of this buffer will be passed to DMA
API’s. While, on QPIC NAND controller, which uses BAM DMA, we read
the controller registers by preparing a BAM command descriptor. This
command descriptor requires the

  - controller register address
  - the DMA address in which we want to store the value read
    back from the controller register.

This command descriptor will be remapped with dma_map_sg
and its DMA address will be passed to DMA API’s. Therefore,
it's required that we also map our register read buffer for
DMA (using dma_map_single). We use the returned DMA address
for preparing entries in our command descriptor.

This patch adds the DMA mapping support for register read
buffer. This buffer will be DMA mapped during allocation
time. Before starting of any operation, this buffer will
be synced for device operation and after operation
completion, it will be synced again for CPU.
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>