dt-bindings: iio: adc: renesas,rzg2l-adc: Document RZ/G3S SoC

dt-bindings: iio: adc: renesas,rzg2l-adc: Document RZ/G3S SoC

Document the ADC IP available on the RZ/G3S SoC. The ADC IP on the RZ/G3S
differs slightly from the one found on the RZ/G2L. The identified
differences are as follows:
- different number of channels (one being used for temperature conversion);
  consequently, various registers differ; the temperature channel
  support was not available for the RZ/G2L variant; the #io-channel-cells
  property was added to be able to request the temperature channel from
  the thermal driver
- different default sampling periods
- the RZ/G3S variant lacks the ADVIC register.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://patch.msgid.link/20241206111337.726244-13-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>