iio: adc: Add support for AD4000

Add support for AD4000 series of low noise, low power, high speed,
successive approximation register (SAR) ADCs.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/356109ac61182f16f2379d5d0cadccfe017f505b.1720810545.git.marcelo.schmitt@analog.com


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