iio: adc: rockchip_saradc: Add support for RK3528

The Successive Approximation ADC (SARADC) in RK3528 uses the v2
controller and support:
- 10-bit resolution
- Up to 1MS/s sampling rate
- 4 single-ended input channels
- Current consumption: 0.5mA @ 1MS/s

Add support for the 4 channels of 10-bit resolution supported by SARADC
in RK3528.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20250227184058.2964204-3-jonas@kwiboo.se


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