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>