mmc: omap_hsmmc: 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 omap_hsmmc_request(), dma_map_sg will be issued before
starting the transfer.  It is optional to use pre_req().  If issuing
pre_req(), post_req() must be called as well.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>