mmc: Add multi-block read support to the generic mmc driver

mmc: Add multi-block read support to the generic mmc driver

This patch adds multi-block read support for the generic MMC
driver. Large reads are broken into chunks of 65535 blocks to
ensure that the code works with controllers having a 16 bit block counter.

This patch results in a significant performance improvement.

Time to read a 45 MB file went from 36 seconds to 9 seconds on Overo

Signed-off-by: Steve Sakoman <steve.sakoman@linaro.org>
Tested-by: Steve Sakoman <steve.sakoman@linaro.org>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>