dt-bindings: usb: add binding for USB GPIO based connection detection driver

It's used to support dual role switch via GPIO when use Type-B
receptacle, typically the USB ID pin is connected to an input
GPIO, and also used to enable/disable device when the USB Vbus
pin is connected to an input GPIO.
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1567070558-29417-4-git-send-email-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb-conn-gpio.txt

+USB GPIO Based Connection Detection
+
+This is typically used to switch dual role mode from the USB ID pin connected
+to an input GPIO, and also used to enable/disable device mode from the USB
+Vbus pin connected to an input GPIO.
+
+Required properties:
+- compatible : should include "gpio-usb-b-connector" and "usb-b-connector".
+- id-gpios, vbus-gpios : input gpios, either one of them must be present,
+	and both can be present as well.
+	see connector/usb-connector.txt
+
+Optional properties:
+- vbus-supply : can be present if needed when supports dual role mode.
+	see connector/usb-connector.txt
+
+- Sub-nodes:
+	- port : can be present.
+		see graph.txt
+
+Example:
+
+&mtu3 {
+	connector {
+		compatible = "gpio-usb-b-connector", "usb-b-connector";
+		type = "micro";
+		id-gpios = <&pio 12 GPIO_ACTIVE_HIGH>;
+		vbus-supply = <&usb_p0_vbus>;
+	};
+};