dt-bindings: iio: adc: Add rockchip,rk3528-saradc variant

dt-bindings: iio: adc: Add rockchip,rk3528-saradc variant

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 a rockchip,rk3562-saradc compatible string for the 4 channels of
10-bit resolution supported by SARADC in RK3528.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20250227184058.2964204-2-jonas@kwiboo.se
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>