Documentation: sound: add rt3261 codec documentation.

Signed-off-by: sugar <sugar.zhang@rock-chips.com>

diff --git a/Documentation/devicetree/bindings/sound/rt3261.txt b/Documentation/devicetree/bindings/sound/rt3261.txt
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+rt3261 audio CODEC
+
+This device supports I2C only.
+
+Required properties:
+
+  - compatible : "rt3261"
+
+  - reg : the I2C address of the device.
+
+  - codec-en-gpio : the enable gpio of codec.
+
+  - spk-num : the number of speaker.
+
+  - modem-input-mode : modem input mode (DIFFERENTIAL/SINGLE_END).
+
+  - lout-to-modem_mode : line out mode.
+
+  - spk-amplify : Gain control for speaker.
+
+Example:
+
+rt3261: rt3261@1c {
+       compatible = "rt3261";
+       reg = <0x1c>;
+       codec-en-gpio = <&gpio3 GPIO_D7 GPIO_ACTIVE_HIGH>;
+       spk-num= <2>;
+       modem-input-mode = <1>;
+       lout-to-modem_mode = <1>;
+       spk-amplify = <2>;
+};