dt-bindings: sound: lpass-cpu: Document DAI subnodes

The lpass-cpu driver now allows configuring the MI2S SD lines
by defining subnodes for one of the DAIs.

Document this in the device tree bindings.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20200425184657.121991-1-stephan@gerhold.netSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/qcom,lpass-cpu.txt

@@ -30,6 +30,8 @@ Required properties:
 - reg			: Must contain an address for each entry in reg-names.
 - reg-names		: A list which must include the following entries:
 				* "lpass-lpaif"
+- #address-cells	: Must be 1
+- #size-cells		: Must be 0
 
 
 
@@ -37,6 +39,20 @@ Optional properties:
 
 - qcom,adsp		: Phandle for the audio DSP node
 
+By default, the driver uses up to 4 MI2S SD lines, for a total of 8 channels.
+The SD lines to use can be configured by adding subnodes for each of the DAIs.
+
+Required properties for each DAI (represented by a subnode):
+- reg			: Must be one of the DAI IDs
+			  (usually part of dt-bindings header)
+- qcom,playback-sd-lines: List of serial data lines to use for playback
+			  Each SD line should be represented by a number from 0-3.
+- qcom,capture-sd-lines	: List of serial data lines to use for capture
+			  Each SD line should be represented by a number from 0-3.
+
+Note that adding a subnode changes the default to "no lines configured",
+so both playback and capture lines should be configured when a subnode is added.
+
 Example:
 
 lpass@28100000 {
@@ -51,4 +67,13 @@ lpass@28100000 {
 	reg = <0x28100000 0x10000>;
 	reg-names = "lpass-lpaif";
 	qcom,adsp = <&adsp>;
+
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	/* Optional to set different MI2S SD lines */
+	dai@3 {
+		reg = <MI2S_QUATERNARY>;
+		qcom,playback-sd-lines = <0 1>;
+	};
 };