iio: adc: Add TI ADS8688

This patch adds support for the Texas Intruments ADS8688 ADC.

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Reviewed-by: Martin Hundebøll <martin.hundeboll@prevas.dk>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>