ath79: fix I2C on GL-AR300M devices

On GL-AR300M Series GPIO17 described as I2C SDA in Device Tree.
Because of GPIO_OUT_FUNCTION4 register was not initialized on start,
GPIO17 was uncontrollable, it always in high state. According to QCA9531
documentation, default setting of GPIO17 is SYS_RST_L. In order to make
GPIO17 controllable, it should write value 0x00 on bits [15:8] of
GPIO_OUT_FUNCTION4 register, located at 0x1804003C address.

Signed-off-by: Ptilopsis Leucotis <PtilopsisLeucotis@yandex.com>

target/linux/ath79/dts/qca9531_glinet_gl-ar300m.dtsi

@@ -72,6 +72,9 @@
 	i2c: i2c {
 		compatible = "i2c-gpio";
 
+		pinctrl-names = "default";
+		pinctrl-0 = <&enable_gpio17>;
+
 		sda-gpios = <&gpio 17 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
 		scl-gpios = <&gpio 16 (GPIO_ACTIVE_HIGH|GPIO_OPEN_DRAIN)>;
 	};
@@ -181,3 +184,9 @@
 		reg = <0x0 0x6>;
 	};
 };
+
+&pinmux {
+	enable_gpio17: pinmux_enable_gpio17 {
+		pinctrl-single,bits = <0x10 0x0000 0xff00>;
+	};
+};