spi: spi-loopback-test: Add test to trigger DMA/PIO mixing

Add a test where a small and a large transfer in a message hit the same
cache line.  This test currently fails on spi-s3c64xx on in DMA mode
since it ends up mixing DMA and PIO without proper cache maintenance.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220916113951.228398-2-vincent.whitchurch@axis.com


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