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>

diff --git a/Documentation/devicetree/bindings/sound/st,stm32-i2s.txt b/Documentation/devicetree/bindings/sound/st,stm32-i2s.txt
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+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>;
+               };
+       };
+};