arm64: dts: rockchip: enable cec pin for rk3399 firefly

Add a pinctrl setting to configure the cec pin to the correct function.
Signed-off-by: Pierre-Hugues Husson <phh@phh.me>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-firefly.dts

@@ -261,6 +261,8 @@ &gmac {
 
 &hdmi {
 	ddc-i2c-bus = <&i2c3>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&hdmi_cec>;
 	status = "okay";
 };