iio: adc: driver for ti adc081s/adc101s/adc121s

    Add Linux device driver for TI single-channel CMOS
    8/10/12-bit analog-to-digital converter with a
    high-speed serial interface.

Signed-off-by: Milan Stevanovic <milan.o.stevanovic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>