dt-bindings: rcar-gen3-phy-usb3: Add r8a774b1 support

Document RZ/G2N (R8A774B1) SoC bindings.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1570531132-21856-5-git-send-email-fabrizio.castro@bp.renesas.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/phy/rcar-gen3-phy-usb3.txt

@@ -8,6 +8,8 @@ need this driver.
 
 Required properties:
 - compatible: "renesas,r8a774a1-usb3-phy" if the device is a part of an R8A774A1
+	      SoC.
+	      "renesas,r8a774b1-usb3-phy" if the device is a part of an R8A774B1
 	      SoC.
 	      "renesas,r8a7795-usb3-phy" if the device is a part of an R8A7795
 	      SoC.