dt: Add bindings documentation for the ADI AXI-I2S controller

This patch adds the devicetree documentation for the ADI AXI-SPDIF audio
controller. The controller has:
* One set of memory mapped register
* Two clocks, one for the memory mapped register interface, one used as the
audio reference clock
* One DMA interface each for the transmit and receive data

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Pawel Moll <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Stephen Warren <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: [e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/adi,axi-i2s.txt

···
+ADI AXI-I2S controller
+
+Required properties:
+ - compatible : Must be "adi,axi-i2s-1.00.a"
+ - reg : Must contain I2S core's registers location and length
+ - clocks : Pairs of phandle and specifier referencing the controller's clocks.
+   The controller expects two clocks, the clock used for the AXI interface and
+   the clock used as the sampling rate reference clock sample.
+ - clock-names : "axi" for the clock to the AXI interface, "ref" for the sample
+   rate reference clock.
+ - dmas: Pairs of phandle and specifier for the DMA channels that are used by
+   the core. The core expects two dma channels, one for transmit and one for
+   receive.
+ - dma-names : "tx" for the transmit channel, "rx" for the receive channel.
+
+For more details on the 'dma', 'dma-names', 'clock' and 'clock-names' properties
+please check:
+	* resource-names.txt
+	* clock/clock-bindings.txt
+	* dma/dma.txt
+
+Example:
+
+	i2s: i2s@0x77600000 {
+		compatible = "adi,axi-i2s-1.00.a";
+		reg = <0x77600000 0x1000>;
+		clocks = <&clk 15>, <&audio_clock>;
+		clock-names = "axi", "ref";
+		dmas = <&ps7_dma 0>, <&ps7_dma 1>;
+		dma-names = "tx", "rx";
+	};