iio: adc: aspeed: Support ast2600 adc.

Make driver to support ast2600 adc device.
- Use shared reset controller
- Complete the vref configure function
- Add the model data for ast2600 adc

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-7-billy_tsai@aspeedtech.com


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