dt-bindings: extcon: convert extcon-usb-gpio.txt to yaml format

Convert binding doc extcon-usb-gpio.txt to yaml format to fix below
warning:
arch/arm64/boot/dts/freescale/imx8mp-msc-sm2s-ep1.dtb: /extcon-usb0:
failed to match any schema with compatible: ['linux,extcon-usb-gpio']

Additional change:
- rename id-gpio to id-gpios
- rename vbus-gpio to vbus-gpios

Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Frank Li <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Chanwoo Choi <[e-mail]>

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

···
-USB GPIO Extcon device
-
-This is a virtual device used to generate USB cable states from the USB ID pin
-connected to a GPIO pin.
-
-Required properties:
-- compatible: Should be "linux,extcon-usb-gpio"
-
-Either one of id-gpio or vbus-gpio must be present. Both can be present as well.
-- id-gpio: gpio for USB ID pin. See gpio binding.
-- vbus-gpio: gpio for USB VBUS pin.
-
-Example: Examples of extcon-usb-gpio node in dra7-evm.dts as listed below:
-	extcon_usb1 {
-		compatible = "linux,extcon-usb-gpio";
-		id-gpio = <&gpio6 1 GPIO_ACTIVE_HIGH>;
-	}
-
-	&omap_dwc3_1 {
-		extcon = <&extcon_usb1>;
-	};

Documentation/devicetree/bindings/extcon/linux,extcon-usb-gpio.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/extcon/linux,extcon-usb-gpio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: USB GPIO Extcon device
+
+maintainers:
+  - Frank Li <[e-mail]>
+
+description:
+  This is a virtual device used to generate USB cable states from the USB ID pin
+  connected to a GPIO pin.
+
+properties:
+  compatible:
+    const: linux,extcon-usb-gpio
+
+  id-gpios:
+    description: gpio for USB ID pin. See gpio binding.
+  vbus-gpios:
+    description: gpio for USB VBUS pin.
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    extcon_usb1 {
+        compatible = "linux,extcon-usb-gpio";
+        id-gpios = <&gpio6 1 GPIO_ACTIVE_HIGH>;
+    };