ARM: dts: lan966x: kontron-d10: fix SPI CS

The pinctrl node was missing which change the pin mux to GPIO mode.
Add it so we don't have to rely on the bootloader to set the correct
mode.

Fixes: 79d83b3a ("ARM: dts: lan966x: add basic Kontron KSwitch D10 support")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230616-feature-d10-dt-cleanups-v1-2-50dd0452b8fe@kernel.org

arch/arm/boot/dts/lan966x-kontron-kswitch-d10-mmt.dtsi

@@ -41,7 +41,7 @@ &flx3 {
 	status = "okay";
 
 	spi3: spi@400 {
-		pinctrl-0 = <&fc3_b_pins>;
+		pinctrl-0 = <&fc3_b_pins>, <&spi3_cs_pins>;
 		pinctrl-names = "default";
 		status = "okay";
 		cs-gpios = <&gpio 46 GPIO_ACTIVE_LOW>;
@@ -79,6 +79,12 @@ sgpio_b_pins: sgpio-b-pins {
 		function = "sgpio_b";
 	};
 
+	spi3_cs_pins: spi3-cs-pins {
+		/* CS# */
+		pins = "GPIO_46";
+		function = "gpio";
+	};
+
 	usart0_pins: usart0-pins {
 		/* RXD, TXD */
 		pins = "GPIO_25", "GPIO_26";