iio: adc: aspeed: Fix the calculate error of clock.

iio: adc: aspeed: Fix the calculate error of clock.

The ADC clock formula is
ast2400/2500:
ADC clock period = PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0] + 1)
ast2600:
ADC clock period = PCLK * 2 * (ADC0C[15:0] + 1)
They all have one fixed divided 2 and the legacy driver didn't handle it.
This patch register the fixed factory clock device as the parent of ADC
clock scaler to fix this issue.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-8-billy_tsai@aspeedtech.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>