tests: drivers: spi: loopback: Fix cache coherency on size9/size24

Make tx_data9 and tx_data24 non-cachable within the 9-bit word size and
24-bit word size functions. This repairs a suspected cache coherency issue
where these buffers are written to cache and therefore not seen correctly
by spi/dma.

Signed-off-by: Jacob Wienecke <jacob.wienecke@nxp.com>