spi: imx: add a device specific prepare_message callback

This is just preparatory work which allows to move some initialisation
that currently is done in the per transfer hook .config to an earlier
point in time in the next few patches. There is no change in behaviour
introduced by this patch.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>