dt-bindings: usb: mtk-xhci: add a optional property to disable u3ports

Add a new optional property to disable u3ports
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/mediatek,mtk-xhci.txt

@@ -38,6 +38,8 @@ Optional properties:
 	mode;
  - mediatek,syscon-wakeup : phandle to syscon used to access USB wakeup
 	control register, it depends on "mediatek,wakeup-src".
+ - mediatek,u3p-dis-msk : mask to disable u3ports, bit0 for u3port0,
+	bit1 for u3port1, ... etc;
  - vbus-supply : reference to the VBUS regulator;
  - usb3-lpm-capable : supports USB3.0 LPM
  - pinctrl-names : a pinctrl state named "default" must be defined