arm64: dts: stratix10: remove 'num-slots' property for dwmmc

Since 'num-slots' had already deprecated, remove the property in
device-tree file.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>