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.