ARM: ux500: Enable support for discard for MMC/SD

By enabling MMC_CAP_ERASE for the SD/MMC devices the mmc block
layer will now act on DISCARD requests.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>