mediatek: mt7986: rename pinctrl to pio in mt7986-u-boot.dtsi

The change from pinctrl to pio was missing in mt7986-u-boot.dtsi and will
cause build failure. Now fix it.

Fixes: f1775996ba9 (mediatek: mt7986: move gpio-controller up and rename pinctrl to pio)
diff --git a/arch/arm/dts/mt7986-u-boot.dtsi b/arch/arm/dts/mt7986-u-boot.dtsi
index 096b973..71e8330 100644
--- a/arch/arm/dts/mt7986-u-boot.dtsi
+++ b/arch/arm/dts/mt7986-u-boot.dtsi
@@ -28,6 +28,6 @@
 	bootph-all;
 };
 
-&pinctrl {
+&pio {
 	bootph-all;
 };