test:drivers:dma:loop_transfer avoid useing flash as dma source address

avoid to use the flash address as dma source,
to avoid flash cachce issue
this is fix to issue #26812
Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>

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

@@ -27,7 +27,7 @@ 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[] =
+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