dt-bindings: reset: renesas,rzg2l-usbphy-ctrl: Document RZ/G2UL USBPHY Control bindings

Add device tree binding document for RZ/G2UL USBPHY Control Device.
RZ/G2UL USBPHY Control Device is identical to one found on the RZ/G2L SoC.
No driver changes are required as generic compatible string
"renesas,rzg2l-usbphy-ctrl" will be used as a fallback.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20220423134601.141975-1-biju.das.jz@bp.renesas.com