imx: imx8mm-cl-iot-gate: enable pinctrl_wdog in SPL

Mark pinctrl_wdog as u-boot,dm-spl to clean up board code,

The set_wdog_reset() function is not necessary as this is handled by
the imx_watchdog.c driver due to the 'fsl,ext-reset-output' property
being set.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/dts/imx8mm-cl-iot-gate-u-boot.dtsi b/arch/arm/dts/imx8mm-cl-iot-gate-u-boot.dtsi
index 433b02c..a7044b6 100644
--- a/arch/arm/dts/imx8mm-cl-iot-gate-u-boot.dtsi
+++ b/arch/arm/dts/imx8mm-cl-iot-gate-u-boot.dtsi
@@ -84,6 +84,10 @@
 	u-boot,dm-spl;
 };
 
+&pinctrl_wdog {
+	u-boot,dm-spl;
+};
+
 &uart3 {
 	u-boot,dm-spl;
 };