dt-bindings: usb-xhci: Document r8a7744 support

Document r8a7744 xhci support. The driver will use the fallback
compatible string "renesas,rcar-gen2-xhci", therefore no driver
change is needed.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -8,6 +8,7 @@ Required properties:
     - "marvell,armada-375-xhci" for Armada 375 SoCs
     - "marvell,armada-380-xhci" for Armada 38x SoCs
     - "renesas,xhci-r8a7743" for r8a7743 SoC
+    - "renesas,xhci-r8a7744" for r8a7744 SoC
     - "renesas,xhci-r8a774a1" for r8a774a1 SoC
     - "renesas,xhci-r8a7790" for r8a7790 SoC
     - "renesas,xhci-r8a7791" for r8a7791 SoC