samples: drivers: stm32u5x octospi enables DMA

Adds the dma transfer for the octoSPI on NOR octo Flash
of the b_u585i_iot02a disco kit.
The channel for the GPDMA is 0-15 (0-7 for DMA1, 8-15 for DMA2)
The channel 12 to 15 are used for GPDMA transfers
to/from external memories.
The request is 41 for the OCTOSPI2.

Signed-off-by: Francois Ramu <[email protected]>

Author: FRASTM

samples/drivers/spi_flash/boards/b_u585i_iot02a.overlay

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2022 STMicroelectronics
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&octospi2 {
+            /* channel 12-15 are for transfers to/from external memories */
+            dmas = <&gpdma1 12 41 0x10480>; /* request 41 for OCTOSPI2 */
+            dma-names = "tx_rx";
+};