tests: dma: loop_transfer: Correct DMA definition of gd32f350r_eval

The overlay file overrides not existing node `dma0`.
Correct it to `dma`.

Signed-off-by: TOKITA Hiroshi <[email protected]>

Author: soburi

tests/drivers/dma/loop_transfer/boards/gd32f350r_eval.overlay

@@ -4,6 +4,6 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-test_dma: &dma0 {
+test_dma: &dma {
 	status = "okay";
 };