phy: rockchip-usb: Add vbus regulator support.

On rockchip devices vbus is supplied by a separate power supply, often
through a regulator. Add support for describing the the regulator in
device-tree following the same convention as several other usb phy's.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>