ASoC: stm32: Document STM32 I2S bindings

Add documentation of device tree bindings for STM32 SPI/I2S.
Signed-off-by: olivier moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/st,stm32-i2s.txt

+STMicroelectronics STM32 SPI/I2S Controller
+
+The SPI/I2S block supports I2S/PCM protocols when configured on I2S mode.
+Only some SPI instances support I2S.
+
+Required properties:
+  - compatible: Must be "st,stm32h7-i2s"
+  - reg: Offset and length of the device's register set.
+  - interrupts: Must contain the interrupt line id.
+  - clocks: Must contain phandle and clock specifier pairs for each entry
+	in clock-names.
+  - clock-names: Must contain "i2sclk", "pclk", "x8k" and "x11k".
+	"i2sclk": clock which feeds the internal clock generator
+	"pclk": clock which feeds the peripheral bus interface
+	"x8k": I2S parent clock for sampling rates multiple of 8kHz.
+	"x11k": I2S parent clock for sampling rates multiple of 11.025kHz.
+  - dmas: DMA specifiers for tx and rx dma.
+    See Documentation/devicetree/bindings/dma/stm32-dma.txt.
+  - dma-names: Identifier for each DMA request line. Must be "tx" and "rx".
+  - pinctrl-names: should contain only value "default"
+  - pinctrl-0: see Documentation/devicetree/bindings/pinctrl/pinctrl-stm32.txt
+
+Optional properties:
+  - resets: Reference to a reset controller asserting the reset controller
+
+The device node should contain one 'port' child node with one child 'endpoint'
+node, according to the bindings defined in Documentation/devicetree/bindings/
+graph.txt.
+
+Example:
+sound_card {
+	compatible = "audio-graph-card";
+	dais = <&i2s2_port>;
+};
+
+i2s2: audio-controller@40003800 {
+	compatible = "st,stm32h7-i2s";
+	reg = <0x40003800 0x400>;
+	interrupts = <36>;
+	clocks = <&rcc PCLK1>, <&rcc SPI2_CK>, <&rcc PLL1_Q>, <&rcc PLL2_P>;
+	clock-names = "pclk", "i2sclk",  "x8k", "x11k";
+	dmas = <&dmamux2 2 39 0x400 0x1>,
+           <&dmamux2 3 40 0x400 0x1>;
+	dma-names = "rx", "tx";
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_i2s2>;
+
+	i2s2_port: port@0 {
+		cpu_endpoint: endpoint {
+			remote-endpoint = <&codec_endpoint>;
+			format = "i2s";
+		};
+	};
+};
+
+audio-codec {
+	codec_port: port@0 {
+		codec_endpoint: endpoint {
+			remote-endpoint = <&cpu_endpoint>;
+		};
+	};
+};