spi: offload-trigger: add PWM trigger driver

Add a new driver for a generic PWM trigger for SPI offloads.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250207-dlech-mainline-spi-engine-offload-2-v8-4-e48a489be48c@baylibre.com


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