drm/i2c: tda998x: add DT documentation

Device-Tree bindings for the NXP TDA998x HDMI transmitter

Required properties;

  - compatible: must be "nxp,tda998x"

Optional properties:

  - interrupts: interrupt number and trigger type

	default: polling

  - pinctrl-0: pin control group to be used for

	screen plug/unplug interrupt.

  - pinctrl-names: must contain a "default" entry.

  - video-ports: 24 bits value which defines how the video controller

	output is wired to the TDA998x input - default: <0x230145>

Example:

	tda998x: hdmi-encoder {

		compatible = "nxp,tda998x";

		reg = <0x70>;

		interrupt-parent = <&gpio0>;

		interrupts = <27 2>;		/* falling edge */

		pinctrl-0 = <&pmx_camera>;

		pinctrl-names = "default";

	};