dt-bindings: usb-xhci: 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/1570531132-21856-6-git-send-email-fabrizio.castro@bp.renesas.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb-xhci.txt

@@ -10,6 +10,7 @@ Required properties:
     - "renesas,xhci-r8a7743" for r8a7743 SoC
     - "renesas,xhci-r8a7744" for r8a7744 SoC
     - "renesas,xhci-r8a774a1" for r8a774a1 SoC
+    - "renesas,xhci-r8a774b1" for r8a774b1 SoC
     - "renesas,xhci-r8a774c0" for r8a774c0 SoC
     - "renesas,xhci-r8a7790" for r8a7790 SoC
     - "renesas,xhci-r8a7791" for r8a7791 SoC