iio: adc: axp20x_adc: put ADC rate setting in a per-variant function

To prepare for a new comer that set a different register with different
values, move rate setting in a function that is specific to each AXP
variant.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>