dt-bindings: usb: renesas_usb3: Document 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/1570531132-21856-7-git-send-email-fabrizio.castro@bp.renesas.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/renesas,usb3-peri.txt

@@ -3,6 +3,7 @@ Renesas Electronics USB3.0 Peripheral driver
 Required properties:
   - compatible: Must contain one of the following:
 	- "renesas,r8a774a1-usb3-peri"
+	- "renesas,r8a774b1-usb3-peri"
 	- "renesas,r8a774c0-usb3-peri"
 	- "renesas,r8a7795-usb3-peri"
 	- "renesas,r8a7796-usb3-peri"