dt-bindings: iio: adc: Add binding documentation for Renesas RZ/G2L A/D converter

Add binding documentation for Renesas RZ/G2L A/D converter block.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210804202118.25745-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>