iio: adc: ad4130: add new supported parts

iio: adc: ad4130: add new supported parts

Add support for AD4129-4/8, AD4130-4, and AD4131-4/8 variants.

The AD4129 series supports the same FIFO interface as the AD4130 but with
reduced resolution (16-bit). The AD4131 series lacks FIFO support, so
triggered buffer functionality is introduced.

The 4-channel variants feature fewer analog inputs, GPIOs, and sparse pin
mappings for VBIAS, analog inputs, and excitation currents. The driver now
handles these differences with chip-specific configurations, including pin
mappings and GPIO counts.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>