dt-bindings: phy: rcar-gen3-phy-usb2: Revise #phy-cells property

To have the detailed property on each PHY specifier, this patch revises
the #phy-cells property.
Signed-off-by: NYoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/rcar-gen3-phy-usb2.txt

@@ -29,7 +29,13 @@ Required properties:
 
 - reg: offset and length of the partial USB 2.0 Host register block.
 - clocks: clock phandle and specifier pair(s).
-- #phy-cells: see phy-bindings.txt in the same directory, must be <0>.
+- #phy-cells: see phy-bindings.txt in the same directory, must be <1> (and
+	      using <0> is deprecated).
+
+The phandle's argument in the PHY specifier is the INT_STATUS bit of controller:
+- 1 = USBH_INTA (OHCI)
+- 2 = USBH_INTB (EHCI)
+- 3 = UCOM_INT (OTG and BC)
 
 Optional properties:
 To use a USB channel where USB 2.0 Host and HSUSB (USB 2.0 Peripheral) are