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

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

It makes sense that if the USB connector is a child of an USB port
providing VBUS supply, there is no need to do it again.
But this does not handle the case where VBUS is controlled by PWR from
USB host controller, without any regulator at all.
Support this by making VBUS pure optional.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20220309093842.113260-1-alexander.stein@ew.tq-group.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>