drm/i2c: tda998x: add DT documentation

Acked-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NJean-Francois Moine <moinejf@free.fr>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

Documentation/devicetree/bindings/drm/i2c/tda998x.txt

+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";
+	};