iio: adc: rockchip_saradc: Add callback functions

Add start, read and power_down callback functions,
which will help in adding new rockchip device support
cleanly.

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>
Link: https://lore.kernel.org/r/20230603185340.13838-2-shreeya.patel@collabora.com


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