iio:adc: Add common code for ADI Sigma Delta devices

Most devices from the Analog Devices Sigma Delta family use a similar scheme for
communication with the device. This includes register access, as well as trigger
handling. But each device sub-family has different features and different
register layouts (some even have no registers at all) and thus it is impractical
to try to support all of the devices by the same driver. This patch adds a
common base library for Sigma Delta converter devices. It will be used by
individual drivers.

This code is mostly based on the three existing Sigma Delta drivers the AD7192,
AD7780 and AD7793, but has been improved for more robustness and flexibility.

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