iio: adc: aspeed: Get and set trimming data.

The ADC controller has a trimming register for fine-tune the reference
voltage. The trimming value comes from the OTP register which will be
written during chip production. This patch will read this OTP value and
configure it to the ADC register when the ADC controller probes and using
dts property "aspeed,trim-data-valid" to determine whether to execute this
flow.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-12-billy_tsai@aspeedtech.com


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