dt-bindings: iio: adc: adi,ad4695: add SPI offload properties

dt-bindings: iio: adc: adi,ad4695: add SPI offload properties

Add a pwms property to the adi,ad4695 binding to specify an optional PWM
output connected to the CNV pin on the ADC.

Also 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,ad4695.h for the cell values to help with
readability since they are arbitrary values.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-13-e48a489be48c@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>