iio: adc: ad7606: add support for writing registers when using backend

Add the logic for effectively enabling the software mode for the
iio-backend, i.e. enabling the software mode channel configuration and
implementing the register writing functions.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Co-developed-by: Angelo Dureghello <adureghello@baylibre.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-9-160df18b1da7@baylibre.com


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