mmc: dw_mmc: set to MMC_CAP_ERASE by default

This flag needs to use the trim/discard/erase commands.
dwmmc controller enables this flag by default.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>