iio: adc: axp20x_adc: Replace adc_en2 flag with adc_en2_mask field

iio: adc: axp20x_adc: Replace adc_en2 flag with adc_en2_mask field

The adc_en2 flag is essentially specific to axp20x-compatible devices
because it hardcodes register values. Replace it with a mask field
so the register value can be specified in device match data.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20230217153404.32481-2-aidanmacdonald.0x0@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>