spi: fsl_dspi: rename num-cs to spi-num-chipselects

The official devicetree bindings specifies spi-num-chipselects as the
name. Use it.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/arch/arm/dts/fsl-ls2080a.dtsi b/arch/arm/dts/fsl-ls2080a.dtsi
index 278daee..72ba525 100644
--- a/arch/arm/dts/fsl-ls2080a.dtsi
+++ b/arch/arm/dts/fsl-ls2080a.dtsi
@@ -93,7 +93,7 @@
 		#size-cells = <0>;
 		reg = <0x0 0x2100000 0x0 0x10000>;
 		interrupts = <0 26 0x4>; /* Level high type */
-		num-cs = <6>;
+		spi-num-chipselects = <6>;
 	};
 
 	qspi: quadspi@1550000 {