dt-bindings: adc: axi-adc: update bindings for backend framework

dt-bindings: adc: axi-adc: update bindings for backend framework

'adi,adc-dev' is now deprecated and must not be used anymore. Hence,
also remove it from being required.

The reason why it's being deprecated is because the axi-adc CORE is now
an IIO service provider hardware (IIO backends) for consumers to make use
of. Before, the logic with 'adi,adc-dev' was the opposite (it was kind
of consumer referencing other nodes/devices) and that proved to be wrong
and to not scale.

Now, IIO consumers of this hardware are expected to reference it using the
io-backends property. Hence, the new '#io-backend-cells' is being added
so the device is easily identified as a provider.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240210-iio-backend-v11-2-f5242a5fb42a@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>