ASoC: stm32: Add DT bindings for SPDIFRX interface

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

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

+STMicroelectronics STM32 S/PDIF receiver (SPDIFRX).
+
+The SPDIFRX peripheral, is designed to receive an S/PDIF flow compliant with
+IEC-60958 and IEC-61937.
+
+Required properties:
+  - compatible: should be "st,stm32h7-spdifrx"
+  - reg: cpu DAI IP base address and size
+  - clocks: must contain an entry for kclk (used as S/PDIF signal reference)
+  - clock-names: must contain "kclk"
+  - interrupts: cpu DAI interrupt line
+  - dmas: DMA specifiers for audio data DMA and iec control flow DMA
+    See STM32 DMA bindings, Documentation/devicetree/bindings/dma/stm32-dma.txt
+  - dma-names: two dmas have to be defined, "rx" and "rx-ctrl"
+
+Optional properties:
+  - resets: Reference to a reset controller asserting the SPDIFRX
+
+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:
+spdifrx: spdifrx@40004000 {
+	compatible = "st,stm32h7-spdifrx";
+	reg = <0x40004000 0x400>;
+	clocks = <&rcc SPDIFRX_CK>;
+	clock-names = "kclk";
+	interrupts = <97>;
+	dmas = <&dmamux1 2 93 0x400 0x0>,
+	       <&dmamux1 3 94 0x400 0x0>;
+	dma-names = "rx", "rx-ctrl";
+	pinctrl-0 = <&spdifrx_pins>;
+	pinctrl-names = "default";
+
+	spdifrx_port: port {
+		cpu_endpoint: endpoint {
+			remote-endpoint = <&codec_endpoint>;
+		};
+	};
+};
+
+spdif_in: spdif-in {
+	compatible = "linux,spdif-dir";
+
+	codec_port: port {
+		codec_endpoint: endpoint {
+			remote-endpoint = <&cpu_endpoint>;
+		};
+	};
+};
+
+soundcard {
+	compatible = "audio-graph-card";
+	dais = <&spdifrx_port>;
+};