mmc: jz4740: add dma infrastructure for data transfers

Until now the MMC driver for JZ4740 SoC was relying on PIO mode only
for data transfers.
This patch allows the use of DMA for data trasnfers in addition to PIO
mode by relying on DMA Engine.

DMA tranfers performance might be further improved by taking advantage
of the asynchronous request capability of the MMC framework.

Signed-off-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>