iio: adc: adi-axi-adc: convert to regmap

Use MMIO regmap interface. It makes things easier for manipulating bits.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231207-iio-backend-prep-v2-8-a4a33bc4d70e@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>