dt-bindings: iio: adc: document RZ/T2H and RZ/N2H ADC

dt-bindings: iio: adc: document RZ/T2H and RZ/N2H ADC

Document the A/D 12-Bit successive approximation converters found in the
Renesas RZ/T2H (R9A09G077) and RZ/N2H (R9A09G087) SoCs.

RZ/T2H has two ADCs with 4 channels and one with 6.
RZ/N2H has two ADCs with 4 channels and one with 15.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>