mmc: block: Allow disabling 512B sector size emulation

This patch adds support for large sector size of 4KB by disabling
emulation.  This patch passes eMMC DATA_SECTOR_SIZE as the logical
block size during mmc_blk_alloc_req.

In order to use this patch for 4KB sector size, ensure that
USE_NATIVE_SECTOR is enabled, partition table is 4KB sector size
aligned and file system block and sector size are 4KB multiples.

Signed-off-by: Saugata Das <saugata.das@linaro.org>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>