dt-binding: usb: add usb-role-switch property

Add a property usb-role-switch to tell the driver that use
USB Role Switch framework to handle the role switch,
it's useful when the driver has already supported other ways,
such as extcon framework etc.

Cc: Biju Das <biju.das@bp.renesas.com>
Cc: Yu Chen <chenyu56@huawei.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/1567070558-29417-2-git-send-email-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/generic.txt

@@ -30,6 +30,10 @@ Optional properties:
 			optional for OTG device.
  - adp-disable: tells OTG controllers we want to disable OTG ADP, ADP is
 			optional for OTG device.
+ - usb-role-switch: boolean, indicates that the device is capable of assigning
+			the USB data role (USB host or USB device) for a given
+			USB connector, such as Type-C, Type-B(micro).
+			see connector/usb-connector.txt.
 
 This is an attribute to a USB controller such as: