test: dma: avoid useing flash as dma source address

avoid to use the flash address as dma source,
to avoid flash cache issue
this is fix to issue zephyrproject-rtos#26812

Signed-off-by: Hake Huang <[email protected]>

Author: hakehuang

tests/drivers/dma/loop_transfer/src/dma.c

@@ -26,8 +26,8 @@ static __aligned(16) char tx_data[64] __used
 static __aligned(16) char rx_data[TRANSFER_LOOPS][RX_BUFF_SIZE] __used
 	__attribute__((__section__(".nocache.dma")));
 #else
-/* pad to times of 8*/
-static const char tx_data[] =
+/* this src memory shall be in RAM to support usingas a DMA source pointer.*/
+static char tx_data[] =
 	"The quick brown fox jumps over the lazy dog ....";
 static __aligned(16) char rx_data[TRANSFER_LOOPS][RX_BUFF_SIZE] = { { 0 } };
 #endif