mmc: Set pref erase size based on size

If available, eMMC stack uses HC_ERASE_GRP_SIZE as the preferred erase
size. As some high capacity eMMC (64MB) reports this size to 512kB, the
discard operations end up taking very long time.

Improve the behaviour by instead calculating the preferred erase size
based on the eMMC size. In this way the discard operations becomes faster.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
[Ulf: Updated changelog and improved comment in code]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>