ASoC: stm32: Add S/PDIF to SAI bindings

Add S/PDIF IEC6958 protocol support to STM32 SAI bindings.
Signed-off-by: olivier moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -45,6 +45,12 @@ SAI subnodes Optional properties:
 	This property sets SAI sub-block as slave of another SAI sub-block.
 	Must contain the phandle and index of the sai sub-block providing
 	the synchronization.
+  - st,iec60958: support S/PDIF IEC6958 protocol for playback
+	IEC60958 protocol is not available for capture.
+	By default, custom protocol is assumed, meaning that protocol is
+	configured according to protocol defined in related DAI link node,
+	such as i2s, left justified, right justified, dsp and pdm protocols.
+	Note: ac97 protocol is not supported by SAI driver
 
 The device node should contain one 'port' child node with one child 'endpoint'
 node, according to the bindings defined in Documentation/devicetree/bindings/