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>

arch/arc/boot/dts/axs10x_mb.dtsi

@@ -101,7 +101,6 @@ ohci@0x60000 {
 		mmc@0x15000 {
 			compatible = "altr,socfpga-dw-mshc";
 			reg = < 0x15000 0x400 >;
-			num-slots = < 1 >;
 			fifo-depth = < 16 >;
 			card-detect-delay = < 200 >;
 			clocks = <&apbclk>, <&mmcclk>;

arch/arc/boot/dts/vdk_axs10x_mb.dtsi

@@ -104,7 +104,6 @@ ps2: ps2@e0017400 {
 		mmc@0x15000 {
 			compatible = "snps,dw-mshc";
 			reg = <0x15000 0x400>;
-			num-slots = <1>;
 			fifo-depth = <1024>;
 			card-detect-delay = <200>;
 			clocks = <&apbclk>, <&mmcclk>;