dt-bindings: iio: adc: adi,ad7606: add SPI offload properties

dt-bindings: iio: adc: adi,ad7606: add SPI offload properties

Add #trigger-source-cells property to allow the BUSY output to be
used as a SPI offload trigger source to indicate when a sample is ready
to be read.

Macros are added to adi,ad7606.h for the cell values to help with
readability since they are arbitrary values.

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/20250403-wip-bl-spi-offload-ad7606-v1-1-1b00cb638b12@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>