mmc: sunxi: Declare ERASE capability

Declare ERASE capability so we can use filesystems with the discard
option and the fstrim tool.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>