dt-bindings: iio: dac: ad3552r: add iio backend support

There is a version of AXI DAC IP block (for FPGAs) that provides
a physical QSPI bus for AD3552R and similar chips, so supporting
spi-controller functionalities.

For this case, the binding is modified to include some additional
properties.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.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-v9-1-f6960b4f9719@kernel-space.org


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