iio: adc: ad7606: Add compatibility to fw_nodes

On the parallel version, the current implementation is only compatible
with id tables and won't work with fw_nodes, this commit intends to fix
it.

Doing so required to declare ad7606_chip_info structures in the .h file
so to make them accessible to all the driver files that can set a
pointer to the corresponding chip as the driver data.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-5-654faf1ae08c@baylibre.com


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