dt-bindings: iio: dac: adi-axi-dac: add ad3552r axi variant

dt-bindings: iio: dac: adi-axi-dac: add ad3552r axi variant

Add a new compatible and related bindigns for the fpga-based
"ad3552r" AXI IP core, a variant of the generic AXI DAC IP.

The AXI "ad3552r" IP is a very similar HDL (fpga) variant of the
generic AXI "DAC" IP, intended to control ad3552r and similar chips,
mainly to reach high speed transfer rates using a QSPI DDR
(dobule-data-rate) interface.

The ad3552r device is defined as a child of the AXI DAC, that in
this case is acting as an SPI controller.

Note, #io-backend is present because it is possible (in theory anyway)
to use a separate controller for the control path than that used
for the datapath.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-wip-bl-ad3552r-axi-v0-iio-testing-v9-2-f6960b4f9719@kernel-space.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>