tests: drivers: spi: spi_loopback: handle rx/tx buffers nocache memory

Place transfer buffers in non-cacheable memory when
CONFIG_NOCACHE_MEMORY=y. This change ensures that DMA transfer
buffers are allocated in non-cacheable memory on platforms where
CONFIG_NOCACHE_MEMORY is enabled. This avoids potential cache
coherence issues that are not handled by the SPI driver.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>