usb: common: usb-conn-gpio: Make VBUS supply optional

If the connector is the child of a USB port and that USB port already
has a VBUS supply attached to it, it would be redundant to require the
connector to have a VBUS supply. In this case, allow the VBUS supply to
be optional.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200806160248.3936771-1-thierry.reding@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>