Make use of the MMC asynchronous request capability to prepare the
next DMA transfer request in parallel with the current transfer.
This is done by adding pre-request and post-request callbacks that are
used by the MMC framework during an active data transfer.

It should help reduce the impact of DMA preparation overhead on the SD
card performance.
Signed-off-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>