mmc: mxs-mmc: Enable MMC_CAP_ERASE

According to the i.MX23/28 reference manuals both mmc interfaces support
the MMC_ERASE command. So enable this capability in the driver to allow
erase/discard/trim requests.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>