dt-bindings: sound: pdm: Document property 'rockchip,path-map'

This is an optional property to describe data path mapping.

Change-Id: I99fb91a5d1be2e3ec9c7edac206760f589949df5
Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>

Documentation/devicetree/bindings/sound/rockchip,pdm.txt

@@ -28,6 +28,22 @@ Required properties:
 	     pinctrl-names. See ../pinctrl/pinctrl-bindings.txt
 	     for details of the property values.
 
+Optional properties:
+- rockchip,path-map: This is a variable length array, that shows the mapping
+  of SDIx to PATHx. By default, they are one-to-one mapping as follows:
+
+   path0 <-- sdi0
+   path1 <-- sdi1
+   path2 <-- sdi2
+   path3 <-- sdi3
+
+  e.g. "rockchip,path-map = <3 2 1 0>" means the mapping as follows:
+
+   path0 <-- sdi3
+   path1 <-- sdi2
+   path2 <-- sdi1
+   path3 <-- sdi0
+
 Example for rk3328 PDM controller:
 
 pdm: pdm@ff040000 {