iio: adc: rzg2l_adc: Add support for channel 8

iio: adc: rzg2l_adc: Add support for channel 8

The ADC on the Renesas RZ/G3S SoC includes an additional channel (channel
8) dedicated to reading temperature values from the Thermal Sensor Unit
(TSU). There is a direct in-SoC connection between the ADC and TSU IPs.

To read the temperature reported by the TSU, a different sampling rate
(compared to channels 0-7) must be configured in the ADM3 register.

The rzg2l_adc driver has been updated to support reading the TSU
temperature.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/20241206111337.726244-11-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>