mmc: jz4740: prepare next dma transfer in parallel with current transfer

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>