iio: stx104: Add IIO support for the ADC channels

iio: stx104: Add IIO support for the ADC channels

The Apex Embedded Systems STX104 features 16 channels of single-ended (8
channels of true differential) 16-bit analog input. Differential input
configuration may be selected via a physical jumper on the device.
Similarly, input polarity (unipolar/bipolar) is configured via a
physical jumper on the device.

Input gain selection is available to the user via software, thus
allowing eight possible input ranges: +-10V, +-5V, +-2.5V, +-1.25V,
0 to 10V, 0 to 5V, 0 to 2.5V, and 0 to 1.25V. Four input gain
configurations are supported: x1, x2, x4, and x8.

This ADC resolution is 16-bits (1/65536 of full scale). Analog input
samples are taken on software trigger; neither FIFO sampling nor
interrupt triggering is supported by this driver.

The Apex Embedded Systems STX104 is primarily an analog-to-digital
converter device. The STX104 IIO driver was initially placed in the DAC
directory because only the DAC portion of the STX104 was supported at
the time. Now that ADC support has been added to the STX104 IIO driver,
the driver should be moved to the more appropriate ADC directory.

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