iio: Add IIO support for the DAC on the Apex Embedded Systems STX104

The Apex Embedded Systems STX104 is a 16-channel 16-bit analog input and
2-channel 16-bit analog output PC/104 card. The STX104 incorporates a
large one mega-sample FIFO.

This driver provides IIO support for the 2-channel DAC on the STX104.
The base port addresses for the devices may be configured via the "base"
module parameter array.

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