dt-bindings: usb: usb251xb: add documentation for voltage supply

Add the optional voltage supply documentation. If not specified the
dummy-regulator is used.
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Richard Leitner <richard.leitner@skidata.com>
Link: https://lore.kernel.org/r/20190917144449.32739-2-m.felsch@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb251xb.txt

@@ -12,6 +12,7 @@ Required properties :
 
 Optional properties :
  - reset-gpios : Should specify the gpio for hub reset
+ - vdd-supply : Should specify the phandle to the regulator supplying vdd
  - skip-config : Skip Hub configuration, but only send the USB-Attach command
  - vendor-id : Set USB Vendor ID of the hub (16 bit, default is 0x0424)
  - product-id : Set USB Product ID of the hub (16 bit, default depends on type)