dt-bindings: iio: adc: Document Renesas RZ/G2UL ADC

Document Renesas RZ/G2UL ADC bindings. RZ/G2UL ADC is almost identical
to RZ/G2L, but it has 2 analog input channels compared to 8 channels
on the RZ/G2L.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220515060337.16513-2-biju.das.jz@bp.renesas.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>