dt-bindings: iio: dac: adi-axi-adc: add ad7606 variant

dt-bindings: iio: dac: adi-axi-adc: add ad7606 variant

A new compatible is added to reflect the specialized version of the HDL.
We use the parallel interface to write the ADC's registers, and
accessing this interface requires to use ADI_AXI_REG_CONFIG_RD,
ADI_AXI_REG_CONFIG_WR and ADI_AXI_REG_CONFIG_CTRL in a custom fashion.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Co-developed-by: Angelo Dureghello <adureghello@baylibre.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-1-160df18b1da7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>