dt-bindings: usb: usb-xhci: Document r8a7742 support

Document r8a7742 xhci support. The driver will use the fallback
compatible string "renesas,rcar-gen2-xhci", therefore no driver
change is needed.
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -7,6 +7,7 @@ Required properties:
     - "marvell,armada3700-xhci" for Armada 37xx SoCs
     - "marvell,armada-375-xhci" for Armada 375 SoCs
     - "marvell,armada-380-xhci" for Armada 38x SoCs
+    - "renesas,xhci-r8a7742" for r8a7742 SoC
     - "renesas,xhci-r8a7743" for r8a7743 SoC
     - "renesas,xhci-r8a7744" for r8a7744 SoC
     - "renesas,xhci-r8a774a1" for r8a774a1 SoC