spi: axi-spi-engine: use message_prepare/unprepare

spi: axi-spi-engine: use message_prepare/unprepare

This modifies the AXI SPI Engine driver to make use of the
message_prepare and message_unprepare callbacks. This separates
the concerns of allocating and freeing the message state from the
transfer_one_message callback.

The main benfit of this is so that future callers of
spi_finalize_current_message() will not have to do manual cleanup
of the state.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-10-cc59db999b87@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>