iio: adc: aspeed: Support battery sensing.

In ast2600, ADC integrate dividing circuit at last input channel for
battery sensing. This patch use the dts property "battery-sensing" to
enable this feature makes the last channel of each adc can tolerance
higher voltage than reference voltage. The offset interface of ch7 will
be separated when enabling the battery sensing mode.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-11-billy_tsai@aspeedtech.com


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