power: supply: smb347-charger: Implement USB VBUS regulator

SMB347 can supply power to USB VBUS, implement the USB VBUS regulator.
USB VBUS needs to be powered for switching OTG-cable USB port into host
mode.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>