iio: adc: Add support for AD7091R-8

Add support for Analog Devices AD7091R-2, AD7091R-4, and AD7091R-8
low power 12-Bit SAR ADCs with SPI interface.
Extend ad7091r-base driver so it can be used by the AD7091R-8 driver.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/09d1d1c4b39cecc528488efac6094233715f5659.1703013352.git.marcelo.schmitt1@gmail.com


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