ASoC: stm32: fix sync property description in SAI bindings

SAI sync property must be described in SAI subnodes
section, as it is a property of child node.
This patch fixes commit 14f0e5f8d97e632695d92f41f2e91d10d8005d47
"ASoC: stm32: Add synchronization to SAI bindings".

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -20,11 +20,6 @@ Required properties:
 
 Optional properties:
   - resets: Reference to a reset controller asserting the SAI
-  - st,sync: specify synchronization mode.
-	By default SAI sub-block is in asynchronous mode.
-	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.
 
 SAI subnodes:
 Two subnodes corresponding to SAI sub-block instances A et B can be defined.
@@ -44,6 +39,13 @@ SAI subnodes required properties:
   - pinctrl-names: should contain only value "default"
   - pinctrl-0: see Documentation/devicetree/bindings/pinctrl/pinctrl-stm32.txt
 
+SAI subnodes Optional properties:
+  - st,sync: specify synchronization mode.
+	By default SAI sub-block is in asynchronous mode.
+	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.
+
 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.