dt-bindings: iio: adc: add IIO backend support

dt-bindings: iio: adc: add IIO backend support

Add the generic io-backends property to the AD7779 binding to enable
support for the IIO backend framework.

Also add the adi,num-lanes property to set the number of lanes used by
AD7779.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Ioana Risteiu <Ioana.Risteiu@analog.com>
Link: https://patch.msgid.link/20250825221355.6214-3-Ioana.Risteiu@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>