dt-bindings: rcar-gen3-phy-usb2: Add r8a77470 support

Document RZ/G1C (R8A77470) SoC bindings.

For RZ/G1C, this driver is used to enable interrupt generation and
initializing timing registers which is part of phy_init code.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

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

 * Renesas R-Car generation 3 USB 2.0 PHY
 
 This file provides information on what the device node for the R-Car generation
-3 and RZ/G2 USB 2.0 PHY contain.
+3, RZ/G1C and RZ/G2 USB 2.0 PHY contain.
 
 Required properties:
-- compatible: "renesas,usb2-phy-r8a774a1" if the device is a part of an R8A774A1
+- compatible: "renesas,usb2-phy-r8a77470" if the device is a part of an R8A77470
+	      SoC.
+	      "renesas,usb2-phy-r8a774a1" if the device is a part of an R8A774A1
 	      SoC.
 	      "renesas,usb2-phy-r8a774c0" if the device is a part of an R8A774C0
 	      SoC.