Add a vbus_gpio field to platform data. This mirrors the device tree
property nvidia,vbus-gpio. This makes the VBUS GPIO handling identical
between booting with board files and device tree; the driver always does
it.

This removes the need for board files to request and initialize the GPIO
early during their boot process, perhaps even before the GPIO driver is
ready to process the request.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>