iio:adc: Add support for AD7766/AD7767

Add support for the AD7766, AD7766-1, AD7766-2, AD7767, AD7767-1, AD7767-2
Analog to Digital converters. It's a family of single channel 24-bit SAR
ADCs. They are all digital interface compatible and the main difference is
the internal decimation rate and analog performance. For communication with
the host processor a SPI interface is used.

In addition the part has a data ready pin that is pulsed for one MCLK cycle
when a conversion has completed and can be used as a IIO trigger.

Datasheets:
	http://www.analog.com/media/en/technical-documentation/data-sheets/AD7766.pdf
	http://www.analog.com/media/en/technical-documentation/data-sheets/AD7767.pdf



Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>