iio:adc:ad7949: Add AD7949 ADC driver family

Compatible with AD7682 and AD7689 chips.
It is a Analog Devices ADC driver 14/16 bits 4/8 channels
with SPI protocol

Datasheet of the device:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD7949.pdf



Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>