Doc/DT: Add DT binding documentation for tpd12s015 encoder

TPD12S015 HDMI level shifter and ESD protection chip

====================================================

Required properties:

- compatible: "ti,tpd12s015"

Optional properties:

- gpios: CT CP HPD, LS OE and HPD gpios

Required nodes:

- Video port 0 for HDMI input

- Video port 1 for HDMI output

Example

-------

tpd12s015: encoder@1 {

	compatible = "ti,tpd12s015";

	gpios = <&gpio2 28 GPIO_ACTIVE_HIGH>,	/* 60, CT CP HPD */

		<&gpio2 9 GPIO_ACTIVE_HIGH>,	/* 41, LS OE */

		<&gpio2 31 GPIO_ACTIVE_HIGH>;	/* 63, HPD */

	ports {

		#address-cells = <1>;

		#size-cells = <0>;

		port@0 {

			reg = <0>;

			tpd12s015_in: endpoint@0 {

				remote-endpoint = <&hdmi_out>;

			};

		};

		port@1 {

			reg = <1>;

			tpd12s015_out: endpoint@0 {

				remote-endpoint = <&hdmi_connector_in>;

			};

		};

	};

};