iio: adc: ad7606: change channel macros parameters

Add the possibility to pass the *_available parameters to the main
macro.
This is a preparation to add the new channels for software mode and
hardware mode in iio backend mode more easily.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-8-160df18b1da7@baylibre.com


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