mmc: dw_mmc: remove the deprecated "supports-highspeed" property

Remvoe the deprecated "supports-highspeed" property.
DWMMC controller will not use this property anymore.

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>