arc: remove num-slots from arc platforms

dwmmc driver deprecated num-slots and plan to get rid
of it finally. Just move a step to cleanup it from DT.

Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Acked-by: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>