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>