dt-bindings: usb: renesas,usb-xhci: Document RZ/V2M support

Document the RZ/V2M SoC bindings.
The RZ/V2M SoC is a little different to the R-Car implementations.
You can access the registers associated with the currently set DRD mode,
therefore as part of init, we have to set the DRD mode to host.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230121145853.4792-2-biju.das.jz@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>