usb: tegra: Add vbus-supply property for host mode PHYs

Document vbus-supply as an optional property for host mode phy-tegra-usb PHYs.
Signed-off-by: NMikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: NStephen Warren <swarren@nvidia.com>
Tested-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/nvidia,tegra20-usb-phy.txt

@@ -48,5 +48,5 @@ Optional properties:
       peripheral means it is device controller
       otg means it can operate as either ("on the go")
 
-Required properties for dr_mode == otg:
+VBUS control (required for dr_mode == host, optional for dr_mode == otg):
   - vbus-supply: regulator for VBUS