iio: adc: ad7380: add support for ad738x-4 4 channels variants

Add support for ad7380/1/2/3-4 parts which are 4 channels
variants from ad7380/1/2/3

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://lore.kernel.org/r/20240528-adding-new-ad738x-driver-v7-7-4cd70a4c12c8@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>