mmc: core: add mmc_get_dma_dir

Add function for determining DMA direction to core.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>