arm64: dts: zte: remove num-slots from zx296718

dwmmc driver deprecated num-slots and plan to get rid
of it finally. Just move a step to cleanup it from DT.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/zte/zx296718.dtsi

@@ -311,7 +311,6 @@ sd0: mmc@1110000 {
 			clock-frequency = <50000000>;
 			clocks = <&topcrm SD0_AHB>, <&topcrm SD0_WCLK>;
 			clock-names = "biu", "ciu";
-			num-slots = <1>;
 			max-frequency = <50000000>;
 			cap-sdio-irq;
 			cap-sd-highspeed;
@@ -336,7 +335,6 @@ sd1: mmc@1111000 {
 			clock-frequency = <167000000>;
 			clocks = <&topcrm SD1_AHB>, <&topcrm SD1_WCLK>;
 			clock-names = "biu", "ciu";
-			num-slots = <1>;
 			max-frequency = <167000000>;
 			cap-sdio-irq;
 			cap-sd-highspeed;