phy: rcar-gen3-usb2: Add vbus-supply to handle VBUS on/off

To handle the VBUS on/off by a regulator driver, this patch adds
regulator APIs calling in the driver and description about vbus-supply
in the rcar-gen3-phy-usb2.txt.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>