spi: Provide per-message prepare and unprepare operations

Many SPI drivers perform setup and tear down on every message, usually
doing things like DMA mapping the message. Provide hooks for them to use
to provide such operations.

This is of limited value for drivers that implement transfer_one_message()
but will be of much greater utility with future factoring out of standard
implementations of that function.

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