iio: ti-ads124s08: Add DT binding documentation

Adding binding documentation for Texas Instruments ADS124S08
and ADS124S06 ADC.

S08 is a 12 channel ADC
S06 is a 6 channel ADC

Datesheet can be found here:
http://www.ti.com/lit/gpn/ads124s08

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>