iio: adc: aspeed: Add func to set sampling rate.

Add the function to set the sampling rate and keep the sampling period
for a driver used to wait the fresh value.
In addition, since the ADC clock is required when initializing the ADC
device, move clk_prepare_enable ahead of the initialization phase.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-9-billy_tsai@aspeedtech.com


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