mediatek: filogic: BPi-R4: wire up pinctrl for pwm-fan

Assign pwm function of PWM0 pin to the pwm-fan.
This is mostly just cosmetics as it basically reflects the default
setting of that pin.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/files-6.6/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi

@@ -297,11 +297,22 @@
 	status = "okay";
 };
 
+&pio {
+	pwm0_pins: pwm0-pins {
+		mux {
+			groups = "pwm0";
+			function = "pwm";
+		};
+	};
+};
+
 &pwm {
 	status = "okay";
 };
 
 &fan {
+	pinctrl-names = "default";
+	pinctrl-0 = <&pwm0_pins>;
 	pwms = <&pwm 0 50000>;
 	status = "okay";
 };