iio: Add IIO support for the Measurement Computing CIO-DAC family

The Measurement Computing CIO-DAC is a family of 16-bit and 12-bit
analog output devices. The analog outputs are from AD660BN/AD7237
converters with each output buffered by an OP-27. Voltage ranges are
configured via physical jumpers on the device.

This driver does not support the devices' simulataneous update mode; the
XFER jumper option should be deselected for all analog output channels.

This driver provides IIO support for the Measurement Computing CIO-DAC
family: CIO-DAC16, CIO-DAC08, and PC104-DAC06. The base port addresses
for the devices may be configured via the base array module parameter.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>