dt-bindings: usb: generic-ehci: Document dr_mode property

dt-bindings: usb: generic-ehci: Document dr_mode property

Document the optional property dr_mode present on both RZ/G2 and
R-Car Gen3 SoCs.

It fixes dtbs_check warning,
'dr_mode' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210630073013.22415-3-biju.das.jz@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>