iio: adc: adi-axi-adc: add support for AD7606 register writing

iio: adc: adi-axi-adc: add support for AD7606 register writing

Since we must access the bus parallel bus using a custom procedure,
let's add a specialized compatible, and define specialized callbacks for
writing the registers using the parallel interface.

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-v4-6-160df18b1da7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>