mmc: mvsdio: Enable MMC_CAP_ERASE

There is no obvious reasons to why mvsdio shouldn't be able to support
erase/trim/discard operations, hence let's set MMC_CAP_ERASE for it.

Cc: Damien Thebault <damien.thebault@vitec.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Damien Thebault <damien.thebault@vitec.com>