imx: imx8mp_evk: 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/imx8mp-evk-u-boot.dtsi b/arch/arm/dts/imx8mp-evk-u-boot.dtsi
index 7aa9083..f43eb62 100644
--- a/arch/arm/dts/imx8mp-evk-u-boot.dtsi
+++ b/arch/arm/dts/imx8mp-evk-u-boot.dtsi
@@ -43,6 +43,10 @@
 	u-boot,dm-spl;
 };
 
+&pinctrl_wdog {
+	u-boot,dm-spl;
+};
+
 &gpio1 {
 	u-boot,dm-spl;
 };