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.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>