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

Add a new binding for using a PWM signal as a trigger source.

The idea here is similar to e.g. "pwm-clock" to allow a trigger source
consumer to use a PWM provider as a trigger source.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
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-v8-3-e48a489be48c@baylibre.com


Signed-off-by: Mark Brown <broonie@kernel.org>