dt-bindings: reset: Document RZ/G2L USBPHY Control bindings

Add device tree binding document for RZ/G2L USBPHY Control Device.
It mainly controls reset and power down of the USB/PHY.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210719121938.6532-4-biju.das.jz@bp.renesas.com
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>