dt-bindings: trigger-source: add generic GPIO trigger source

dt-bindings: trigger-source: add generic GPIO trigger source

Inspired by pwm-trigger, create a new binding for using a GPIO
line as a trigger source.

Link: https://lore.kernel.org/linux-iio/20250207-dlech-mainline-spi-engine-offload-2-v8-3-e48a489be48c@baylibre.com/
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/c4a3a7c828c711b439d1893271b8376823176ea6.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>