iio: adc: rockchip_saradc: replace custom logic with devm_reset_control_get_optional_exclusive

devm_reset_control_get_optional_exclusive does what this driver is
trying to do in its probe function, therefore let's switch over to that
subsystem function.

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240223-saradcv2-chan-mask-v1-3-84b06a0f623a@theobroma-systems.com


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