mmc: dw_mmc: remove the deprecated "num-slots"

'num-slots' property had already deprecated.
Remove the 'nom-slots' property that is kept to maintain the compatibility.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>