iio: adc: rockchip_saradc: Add support for RK3588

Add new start and read functions to support rk3588 device.
Also, add a device compatible string for the same.

Signed-off-by: Simon Xue <xxm@rock-chips.com>
Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230603185340.13838-3-shreeya.patel@collabora.com


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