iio: dac: adi-axi-dac: add registering of child fdt node

Change to obtain the fdt use case as reported in the
adi,ad3552r.yaml file in this patchset.

The DAC device is defined as a child node of the backend.
Registering the child fdt node as a platform devices.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-wip-bl-ad3552r-axi-v0-iio-testing-v9-8-f6960b4f9719@kernel-space.org


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