dt-bindings: iio: adc: ad7768-1: add trigger-sources property

dt-bindings: iio: adc: ad7768-1: add trigger-sources property

In addition to GPIO synchronization, The AD7768-1 also supports
synchronization over SPI, which use is recommended when the GPIO
cannot provide a pulse synchronous with the base MCLK signal. It
consists of looping back the SYNC_OUT to the SYNC_IN pin and send
a command via SPI to trigger the synchronization.

Introduce the 'trigger-sources' property to enable SPI-based
synchronization via SYNC_OUT pin, along with additional optional
entries for GPIO3 and DRDY pins.

Also create #trigger-source-cells property to differentiate the trigger
sources provided by the ADC. To improve readability, create a
adi,ad7768-1.h header with the macros for the cell values.

While at it, add description to the interrupts property.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: David Lechner <dlechner@baylirbe.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/713fd786010c75858700efaec8bb285274e7057e.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>