dt-bindings: mfd: mediatek: Add MT6397 Pin Controller

The MT6397 mfd includes a pin controller. Add binding
a description for it.
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/mt6397.txt

@@ -60,6 +60,11 @@ Optional subnodes:
 		- compatible: "mediatek,mt6323-pwrc"
 	For details, see ../power/reset/mt6323-poweroff.txt
 
+- pin-controller
+	Required properties:
+		- compatible: "mediatek,mt6397-pinctrl"
+	For details, see ../pinctrl/pinctrl-mt65xx.txt
+
 Example:
 	pwrap: pwrap@1000f000 {
 		compatible = "mediatek,mt8135-pwrap";