mmc: sdhci: add support for pre_req and post_req

pre_req() runs dma_map_sg() post_req() runs dma_unmap_sg.
If not calling pre_req() before sdhci_request(), request()
will prepare the cache just like it did it before.
It is optional to use pre_req() and post_req().

Change-Id: I32cabddc4ec824515fa9ba4ff0ad7bce3679a3ef
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Patch-mainline: linux-mmc @ 16/04/11,  09:43
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>