spi: add devm_spi_optimize_message() helper

Merge series from David Lechner <dlechner@baylibre.com>:

In the IIO subsystem, we are finding that it is common to call
spi_optimize_message() during driver probe since the SPI message
doesn't change for the lifetime of the driver. This patch adds a
devm_spi_optimize_message() helper to simplify this common pattern.

drivers/spi/spi.c

@@ -4384,6 +4384,7 @@ int devm_spi_optimize_message(struct device *dev, struct spi_device *spi,
 
 	return devm_add_action_or_reset(dev, devm_spi_unoptimize_message, msg);
 }
+EXPORT_SYMBOL_GPL(devm_spi_optimize_message);
 
 /**
  * spi_async - asynchronous SPI transfer