MIPS: DTS: Remove num-slots from Pistachio SoC

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

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: James Hartley <james.hartley@sondrel.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16741/Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/boot/dts/img/pistachio.dtsi

@@ -805,7 +805,6 @@ sdhost: mmc@18142000 {
 		pinctrl-0 = <&sdhost_pins>;
 		pinctrl-names = "default";
 		fifo-depth = <0x20>;
-		num-slots = <1>;
 		clock-frequency = <50000000>;
 		bus-width = <8>;
 		cap-mmc-highspeed;