iio: adc: aspeed: Restructure the model data

iio: adc: aspeed: Restructure the model data

This patch refactors the model data structure to distinguish the
function form different versions of aspeed ADC.
- Rename the vref_voltage to vref_fixed_mv and add vref_mv driver data
When driver probe will check vref_fixed_mv value and store it to vref_mv
which isn't const value.
- Add num_channels
Make num_channles of iio device can be changed by different model_data
- Add need_prescaler flag and scaler_bit_width
The need_prescaler flag is used to tell the driver the clock divider needs
another Prescaler and the scaler_bit_width to set the clock divider
bitfield width.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-3-billy_tsai@aspeedtech.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>