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

dwmmc controller is supporting only one slot per a IP.
Even though DWMMC IP is provided the multiple slot, but there is no
usage in real world.
In mmc subsystem, not allow the multiple slot concept.
Then "num-slots" property is deprecated.

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