dt-bindings: usb-xhci: Add r8a77990 support

This patch adds r8a77990 xhci support. The driver will use
the fallback compatible string "renesas,rcar-gen3-xhci".
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@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

@@ -14,6 +14,7 @@ Required properties:
     - "renesas,xhci-r8a7795" for r8a7795 SoC
     - "renesas,xhci-r8a7796" for r8a7796 SoC
     - "renesas,xhci-r8a77965" for r8a77965 SoC
+    - "renesas,xhci-r8a77990" for r8a77990 SoC
     - "renesas,rcar-gen2-xhci" for a generic R-Car Gen2 or RZ/G1 compatible
       device
     - "renesas,rcar-gen3-xhci" for a generic R-Car Gen3 compatible device