[nrf fromtree] 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 <[email protected]>
(cherry picked from commit cc4e2f5)

Author: djiatsaf-st

tests/drivers/spi/spi_loopback/src/spi.c

@@ -717,8 +717,8 @@ static struct k_thread thread[3];
 static K_SEM_DEFINE(thread_sem, 0, 3);
 static K_SEM_DEFINE(sync_sem, 0, 1);
 
-static uint8_t __aligned(32) tx_buffer[3][32];
-static uint8_t __aligned(32) rx_buffer[3][32];
+static uint8_t __aligned(32) tx_buffer[3][32] __NOCACHE;
+static uint8_t __aligned(32) rx_buffer[3][32] __NOCACHE;
 
 atomic_t thread_test_fails;