iio: adc: adi-axi-adc: add platform children support

iio: adc: adi-axi-adc: add platform children support

This is a preparation for the next commit adding support for register
read and write functions on AD7606.
Since sometimes a bus will be used, it has been agreed during ad3552's
driver implementation that the device's driver bus is the backend, whose
device node will be a child node.
To provide the special callbacks for setting the register, axi-adc needs
to pass them to the child device's driver through platform data.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-5-160df18b1da7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>