iio: adc: ad7192: Add AD7194 support

iio: adc: ad7192: Add AD7194 support

Unlike the other AD719Xs, AD7194 has configurable channels. The user can
dynamically configure them in the devicetree.

Add sigma_delta_info member to chip_info structure. Since AD7194 is the
only chip that has no channel sequencer, num_slots should remain
undefined.

Also modify config AD7192 description for better scaling.

Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Link: https://lore.kernel.org/r/20240514120222.56488-7-alisa.roman@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>