iio: adc: sc27xx: Add raw data support

The headset device will use channel 20 of ADC controller to detect events,
but it needs the raw ADC data to do conversion according to its own formula.

Thus we should configure the channel mask separately and configure channel
20 as IIO_CHAN_INFO_RAW, as well as adding raw data read support.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>