iio: adc: ad7476: Add support for TI ADS786X ADCs

Add support for Texas Instruments ADS7866, ADS7867 and ADS7868
8/10/12 bit Single channel ADC.

Datasheet: http://www.ti.com/lit/ds/symlink/ads7868.pdf



Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>