dt-bindings: power: supply: smb347-charger: Document USB VBUS regulator

SMB347 can supply power to USB VBUS, which is required by OTG-cable
devices that want to switch USB port into the host mode. Add USB VBUS
regulator properties.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>